xpepsigirl:
Your problem is similar to the one discussed in this thread:
Library Parts Not Working Properly
You need to go through the skylight's parameters to find the missing fill and select one that is part of your PLN file.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David