Hey @Astadion!
What version of UE5 are you currently using? Also, have you tried using Visual Studio 2019 and/or reinstalling/repairing Visual Studio?
Since this error happened immediately after adding a plugin, the plugin may be looking for something that is missing or not set up yet. Plugins purchased on the marketplace should work even without C++ and you can create C++ in a blueprint project so that should not (in theory) be an issue (especially since VaRest only provides blueprints). Because of this, I would check and see if that plugin needs an update and is installed correctly.
You can find more information in this previous thread:
ERROR: Stage Failed. Missing receipt. Check that this target has been built
As an added note, the following error indicates you should try and build your project from visual studio.
If all else fails I would definitely reach out to the developer of the plugin for help.
I hope one of the above solutions works for you!