Invalid BSP error while publishing an add-on?
Don't worry, you're not alone!

How?
v1.13.2 inadvertently broke the Workshop Publisher functionality due to a bug that slipped through, and every single selected file will return "Invalid BSP" error, even for VPK format, the issue is not with your add-on! (https://github.com/nmrih/source-game/issues/1416)

Workaround:
Before a hotfix ships, you can switch to the temporary previous beta branch on Steam, this will revert your build to v1.13.1 which includes a functional publisher. Be sure to revert back to the main branch when you're done, as a latest version of the client is required to join active servers!