If you use something like this, it should at least make debugging simpler. Then substitute the code I've provided with this line:
Application.Workbooks.Open Filename:=ThisWorkbook.Path & "\UNIT" & "\MARINER"
Declare the variables with your other variables. I'm understanding your question to be that "Mariner" is the file name. If it's not, you will need to change the fileNPath.
Dim path As String, fileNPath As String
'add these two values above your code
path = ActiveWorkbook.path
fileNPath = path & "\UNIT\MARINER.xlsm"
Application.Workbooks.Open fileName:=fileNPath 'Change this line
Once you've done this, you can see the values for the file path in debug mode where the version you have won't have a value until the line that isn't working anyway. So this way, you can SEE what is going on before you try to USE it.
note: To have a macro enabled workbook, it must be .xlsm, not .xlsx
Copy as patha macro-enabled workbook from my Documents folder and pasted that into your code then removed.xlsmfrom the string. Worked fine."\MARINER\filenam.xlsm"?