Does a removed version of Catalina use storage that generates a "low volume storage message
The first thing is to look at your storage allocations.

Open system information and click on the storage tab. If it shows a blank area - just quit all your open apps and set the machine to not sleep for an hour and return



Once the space is there, if you have purgeable data - that will go away when you need it (and if you're reading things correctly, you'll purge with that little space). If you don't have data that's able to be purged, click on the manage button to decide which bucket needs cleaning. 

It's unlikely you have a second OS sitting, but it is possible.