The primary key for the table 225 "Postal Code" is "Code","City" and you can't do the action which you wrote.
1 error - you need to use Lookup-action instead of validate.
2 error - you can't write table relation "Postal Code".Code because you are trying to store the "County" field and it is out of the PK.
2 error - write something like:
var PostalCode record "Post Code";
PostalCodesList page "Post Codes"
PostalCode.SETRANGE|SETFILTER - write filters here..
IF PostalCodesList.RUNMODAL = ACTION::LookupOK then begin
CountyNo := PostalCodes.County;