Insterting key-value pair to public postgreSQL table from node server

I don't know much of a pSQL, however the behaviour that i want is to:

insert a key (string type) - value(string type) into a public table (named stats)

Code i am using is like this

let value = await db.query({
                text: "INSERT INTO public.stats (key,value) values ($1,$2) ON CONFLICT (key) DO UPDATE SET value = $2;",
                values: [wantedStamp,inDb] 
            });
            const rep: Reply_ip_get_background_image_timestamp = {
                name: wantedStamp,
                date: inDb
            };
            res.json(rep).status(200);

Where wantedStamp is a string variable declared before, inDb is the string form of date. When i try to use this i get the error:

error: there is no unique or exclusion constraint matching the ON CONFLICT specification
...some modeule errors amd than...
length: 148,
  severity: 'ERROR',
  code: '42P10',
  detail: undefined,
  hint: undefined,
  position: undefined,
  internalPosition: undefined,
  internalQuery: undefined,
  where: undefined,
  schema: undefined,
  table: undefined,
  column: undefined,
  dataType: undefined,
  constraint: undefined,
  file: 'plancat.c',
  line: '842',
  routine: 'infer_arbiter_indexes'

EDIT------ Now my code is like

let value = await db.query({
                text: "INSERT INTO stats (key,value) VALUES($1,$2) ON CONFLICT (key) WHERE ((key)::text = $1::text) DO UPDATE SET value = $2;",
                values: [wantedStamp,inDb] 
            });
            const rep: Reply_ip_get_background_image_timestamp = {
                name: wantedStamp,
                date: inDb
            };

but it still doesn't work ...