ODK Validate Errors:

Welcome to the ODK forum, @mattload! We're glad you're here. When you get a chance, please introduce yourself on this forum thread. I'd also encourage you to add a real picture as your avatar because it helps build community!

Is this a problem encountered when using Aggregate? Or are you checking a form with XLSForm Online?

If you are able to upload your survey file here, it will help with trouble shooting.

You may also find helpful discussions in other forum threads that mention the error you encountered by searching unsupported construct [filter expression] or other key phrases:
https://forum.getodk.org/search?q=unsupported%20construct%20%5Bfilter%20expression%5D