What's the error message?

The code docmd.runsql s . stops and does not continue updating the testresultT

Try putting:

MsgBox S

before

DoCmd.RunSQL S

and verify what is being passed to RunSQL.

Other than that, we'll need the actual error number and/or statement that pops up

Error 3144 during execution. The UPDATE instruction contains a syntax error.

wild shot here try this one.
Dim S As String

Dim strGrade As String

'Convert Grade to SQL-safe decimal format
strGrade = Replace(CStr(Grade), ",", ".")

S = "UPDATE TestResultT " & _
    "SET [Grade] = " & strGrade & " " & _
    "WHERE [TestResultID] = " & TestResultID & ";"

Debug.Print S   'Check the SQL in the Immediate Window

CurrentDb.Execute S, dbFailOnError

MsgBox "Test is compleet! U heeft een score van " & _
       Round(Grade, 1) * 100 & "%"

DoCmd.Close acForm, "TestTakerF"

Juan Thank you finaly it's working thanks again