If you've just installed the updated LABS plugin and you're still finding the old UI within FL Studio, along with an Error 2, and you've already tried the steps in the guide here, then you can follow this guide to be certain you're finding the right location to remove the old plugin file.
1) Open the FL Studio Plugin Manager by going to Add > Manage Plugins.
2) In the bottom right corner, type "LABS" into the box labeled Find.
3) Double click the LABS name in the list, checking all entries available.
4) Navigate to the path listed on the left side panel within your computer's file system and delete the "LABS.dll", "LABS (64-bit).dll", or "LABS (32-bit).dll" files that may be present.
5) Within FL Studio, open the plugin manager again and rescan plugins.
6) If done correctly you should see the "LABS" plugin appear in the list when searching. When double clicking you should see it's path listed as C:\Program Files\Common Files\VST3\Spitfire Audio.