Quick summary

Imagen can't read your Lightroom catalog while Lightroom is open.

Error message

We can't read the catalog. Close Lightroom Classic. See this Adobe guide, and try again.

How to fix this

1. Close Lightroom Classic completely. Don't just minimize it.
2. Wait a few seconds, and then try again.
3. Delete the .lrcat.lock file if you get the error again.
4. Follow Adobe's catalog lock guide for more info.