I've had this happen before, and Trond is right.
What this message USUALLY means, is that there are some custom terminal types in the PLC module, but ACADE can't find them anywhere. This usually happens for one of two reasons: 1: The PLC module is in a drawing from someone else (i.e. didn't originate with you/your company). Or 2: You upgraded to a new version of ACADE.
However, since you're trying to create a new module, it's more likely that what's happening is that somehow ACADE lost track of the wdmbblks file, or that the file was somehow moved or deleted. When this happened to me, it was because I was moving to a shared-on-the-network system and I didn't know about the existence of this file. I saw then basically what you're seeing now, except that it was everyone BUT me getting that error message - they were all using their default wdmbblks file to look at PLCs that were completely made out of custom modules. Only my copy of the wdmbblks file had the custom terminal types defined.
Here's what I would do:
1. Follow Trond's suggestion. If ACADE finds a wdmbblks file, open it and see what it says. I attached a screenshot of mine so you can compare. (Note that I have some custom terminal types at the bottom.)
2. If ACADE can't find the file, then do a search to see if it exists at all. If you share folders on the network be sure to search there too.
3. If the file is gone, post back and one of us can at least share a copy of the default one with you. (Or, you can reinstall the software to restore it.)
My wdmbblks file is in the samle place as Trond's, so it looks like the default location for this file is in the default support folder at users/you/appdata/roaming/autodesk.......support. Let us know how it goes.

Jim Seefeldt
Electrical Engineering Technician