Undefined union classes

[HEWalker]

Hello, I'm trying to use WebVowl to visualise an ontology I made in Protégé. When I import the ontology into WebVowl it generates a lot of undefined union classes that make it impossible to export as TTL. It gives the message:

Warning:
Can not export ontology
Reason:
Detected unsupported ontology axioms, (e.g. owl:Union)
Action:
Ontology is not exported

I have tried manually editing to remove these on a small sample of the ontology and this makes exporting possible but is there a way to prevent the creation of these in the first place and show only the properties linked directly to their domains and ranges?

Any other work arounds would also be welcome.

I'm a PhD student working on a digital humanities project but I've taught myself ontology stuff to do it and don't have much IT background knowledge so apologies if this is not the place to ask!

TIA