Can someone explain what this means and how to fix it? (If I can?)
Maybe this Steam article can help: https://support.steampowered.com/kb_article.php?ref=8676-EFLX-7453
Also try restarting your computer.
By chance, did you only ever play stage 2? Did you have an original key?
While I mainly played Stage 2 I do have an original key.
and you have went into the settings and selected opt in beta?
Yes, it is currently Evolve Stage 2 [legacyevolve]
Did you try verifying the files? That usually fixes issues with missing files by redownloading them.
If you don’t know how, you can do that by right-clicking the game in your library and going to Properties > Local FIles > Verify Integrity of Game Files
How long has it been since you last played Legacy? Or did this occur immediately after a fresh download?