Strange Problem with Button to mark check box

I suggest that you use the table name like [_DBID_INVOICE_HEADER].  It's possible that you have used the application ID and hence the  bug9w4puk is incorrect.

You get that from the advanced settings for the invoice header table.  In fact the best practice is to always use a reference to the table as opposed to hard coding the Table ID or even the application ID because that way when you copy the app the button will still work.  

var text url = URLRoot() & "db/" & Dbid() & "?a=API_EditRecord" &
    "&rid=" & [Record ID#] &
    "&apptoken=d7s84zwcx5qf79csb72q6wrdjkd" &
    "&_fid_20=1" &
    "&rdr=" & //redirect parameter
    //URLEncode("https://exteriorsutah.quickbase.com/db/bug9w4puk"); URL meaning
    URLEncode(URLRoot() & "db/" & [_DBID_INVOICE_HEADER] & "?a=er&rid=" & [Related Invoice Header]); // Takes them to the invoice header in view mode, dr is view mode er is edit mode
    "<a class='Vibrant Primary' style=\"width:160px; text-align: center;\" href='" & $url & "'>Select for Pay</a>"