Looks like it's not picking up a UserID
UserID=ANDGroupID=1

That error is telling you there is no value in Forms!MainMenuF!txtUserID. It looks like, from your second image, that you are trying to get the value before the Main Menu is loaded.

In addition to what the other guys said above, pay attention to spaces.  They matter in this case.  It looks like you need a space before AND.

The best way to troubleshoot the formula is to message box it sometimes to make sure it is working properly.
Temporarily put in this right after X=......

     MsgBox X
     Exit Sub


Make sure you are using the correct form.
I see that you are using the LoginF when you get the error.  The code in the 3rd picture is trying to refer to Forms!mainmenuf!txtUserID

I'm assuming here but I don't think you would have that value set yet on your mainmenuf yet if you are logging in.
A clue for me is that you don't have the CamelCase in your form name either.  Check spelling.

Kevin  Sorry I just saw your response.  Had the pics open in another screen when I replied and missed yours.

LOL Scott I do that often.

Thank you all,

Fixed the CamelCase and space before AND.  Same error.  
Deleting the On Current Event of the MainMenuF allows it move on from the LoginF and the UserID shows on the MainMenuF.


Assigning the same code to an On Click button in the MainMenuF works.

"If IsUserInGroup(1) Then
Me.FormulationSelectionCombo.Visible = False

    Else
Me.FormulationSelectionCombo.Visible = True

End If
End Sub

Just to confirm what is the code behind the login button? Could you also share screenshot of the 3 tables?  Users groups userxgroups. Are any of the number fields short text? Just a few ideas we might find an error in

Sorry, images attached above, I should have explained, I still want the On Current to work, assigning the code to a Command Button was only a means to test the Public code once in the MainMenuF.

Kind Regards
Pino

On the login form you are referring to the main menu before its even set or open. Change the forms!mainmenu stuff to the name of that combobox on your loginf

Thank you Adam,

You got it regarding the reference to the MainMenuF before being open.

Created a txtUserID on the LoginF form and changed the code to the attached.  All working as should as long as the LoginF is open, I can work around that. However, any suggestions to the IsUserInGroup code to have it referencing off the MainMenuF and close the LoginF?

Thank you for your help.

Regards
Pino