Looks like you have the 2018 Macmini8,1. What is the size of the SSD and how much memory does it have?
Usually, when the system is running low on available memory, it caches memory content onto the drive, but if you are also running low on drive space, that error is bound to come up. Since the 2018 Mac mini is difficult to upgrade (RAM and SSD), your easiest option is to remove files from your internal drive. It’s really easy to add external drives to the mini, so you can easily move much of your data to external storage.