This is likely to be caused by a movement of your OBS installation to a new system - or if you reinstalled windows and copied over your OBS folder and the one in appdata/roaming. The issue might be you have chosen a new username on the new system. If so, read on.
Easy to fix:
Go to C:\users\<YOUR USERNAME>\AppData\Roaming\obs-studio\
You should see global.ini and users.ini in there. If that's correct, open global.ini with an editor. Scroll down to the end where you find the section
[Locations]
Change the username (the one after C:\\Users\\) to reflect your current username for all three lines. Save the ini and you should be fine.
In case you forgot: in Windows, OBS uses 2 locations - 1. the installation directory 2. you Appdata\Roaming\obs-studio space. Make sure you backupped both.