# dnf list installed | grep postgis
postgis31_10.x86_64                           3.1.7-1.rhel8                                       @rhel8-pgdg10          
postgis31_10-client.x86_64                    3.1.7-1.rhel8                                       @rhel8-pgdg10          
postgis31_10-devel.x86_64                     3.1.7-1.rhel8                                       @@commandline          
postgis31_10-utils.x86_64                     3.1.7-1.rhel8                                       @@commandline

# dnf list installed | grep postgresql
VRTSnbpostgresqlagent.x86_64                  9.1.0.0-1620848511                                  @System                
postgresql.x86_64                             10.17-2.module+el8.5.0+11838+8dca950a               @rhel8-appstream       
postgresql-server.x86_64                      10.17-2.module+el8.5.0+11838+8dca950a               @rhel8-appstream       
postgresql10.x86_64                           10.22-1PGDG.rhel8                                   @rhel8-pgdg10          
postgresql10-contrib.x86_64                   10.22-1PGDG.rhel8                                   @rhel8-pgdg10          
postgresql10-libs.x86_64                      10.22-1PGDG.rhel8                                   @rhel8-pgdg10          
postgresql10-server.x86_64                    10.22-1PGDG.rhel8                                   @rhel8-pgdg10
# dnf list installed | grep postgis
postgis31_10.x86_64                           3.1.7-1.rhel8                                       @rhel8-pgdg10          
postgis31_10-client.x86_64                    3.1.7-1.rhel8                                       @rhel8-pgdg10          
postgis31_10-devel.x86_64                     3.1.7-1.rhel8                                       @@commandline          
postgis31_10-utils.x86_64                     3.1.7-1.rhel8                                       @@commandline

# dnf list installed | grep postgis
postgis31_10.x86_64                           3.1.7-1.rhel8                                       @rhel8-pgdg10          
postgis31_10-client.x86_64                    3.1.7-1.rhel8                                       @rhel8-pgdg10          
postgis31_10-devel.x86_64                     3.1.7-1.rhel8                                       @@commandline          
postgis31_10-utils.x86_64                     3.1.7-1.rhel8                                       @@commandline

(My colleague manually downloaded those latter two packages from... somewhere)

When we try to CREATE EXTENSION postgis; we get the following error

ERROR:  could not access file "$libdir/postgis-3": No such file or directory

Our PostgreSQL installation location in /usr/pgsql-10/ and inside /usr/pgsql-10/lib are the .so files for the extension (e.g. postgis-3.so). Inside /usr/pgsql-10/share/extension are postgis*.control files which contain:

# postgis extension
comment = 'PostGIS geometry and geography spatial types and functions'
default_version = '3.1.7'
module_pathname = '$libdir/postgis-3'
relocatable = false

When I tried adding a libdir=/usr/pgsql-10/lib line to the file the error was ERROR: unrecognized parameter "libdir" in file "/usr/share/pgsql/extension/postgis.control" but when we replaced the path in module_pathname to include /usr/pgsql-10/lib we just got the same error as before ERROR: could not access file "$libdir/postgis-3": No such file or directory

This server is managed by our IT who helped install some of the postgis packages so I'm not sure if there's somehow a package universe mismatch here.

# dnf list installed | grep postgis
postgis31_10.x86_64                           3.1.7-1.rhel8                                       @rhel8-pgdg10          
postgis31_10-client.x86_64                    3.1.7-1.rhel8                                       @rhel8-pgdg10          
postgis31_10-devel.x86_64                     3.1.7-1.rhel8                                       @@commandline          
postgis31_10-utils.x86_64                     3.1.7-1.rhel8                                       @@commandline

(My colleague manually downloaded those latter two packages from... somewhere)

When we try to CREATE EXTENSION postgis; we get the following error

ERROR:  could not access file "$libdir/postgis-3": No such file or directory

Our PostgreSQL installation location in /usr/pgsql-10/ and inside /usr/pgsql-10/lib are the .so files for the extension (e.g. postgis-3.so). Inside /usr/pgsql-10/share/extension are postgis*.control files which contain:

# postgis extension
comment = 'PostGIS geometry and geography spatial types and functions'
default_version = '3.1.7'
module_pathname = '$libdir/postgis-3'
relocatable = false

When I tried adding a libdir=/usr/pgsql-10/lib line to the file the error was ERROR: unrecognized parameter "libdir" in file "/usr/share/pgsql/extension/postgis.control" but when we replaced the path in module_pathname to include /usr/pgsql-10/lib we just got the same error as before ERROR: could not access file "$libdir/postgis-3": No such file or directory. Also tried installing other non-PostGIS extensions and got similar could not access "$libdir/extension" errors.

This server is managed by our IT who helped install some of the postgis packages so I'm not sure if there's somehow a package universe mismatch here.

Some further context after reading this answer, the PostgreSQL data directory is mounted at a different location for space reasons (/pgdata), so maybe something got missed in that configuration change?