That's because you are trying to set the backstyle property of a control
that doesn't have one, such as a command button, for instance. You need to
make some provision in your code to set the property of the desired controls
only. Looking at the names of the controls in your first post, it looks like
the following could work:
For Each ctl In ProjectenForm.Controls
If Left(ctl.Name, 2) = "Eg" Or Left(ctl.Name, 2) = "Mg" Then
ctl.BackStyle = fmBackStyleOpaque
End If
Next
provided that the name of the control causing the error doesn't start with
Eg or Mg.
HTH,
Nikos