Guilty should be the local stored Time Machine snapshots, that are not reported as used space to user but it is reported to applications. “Thinning” them should resolve the issue. A df from console should give you the tip: your main partition is almost full in “Capacity” column.
With CleanMyMac (I’m not fan of this program, but has some helpful tools), go to Maintenance → Reduce Time Machine Snapshots.
With command line, online search for tmutil listlocalsnapshots and follow any of the tutorials available.
Howard Oakley provides a lot of insight into Time Machine backups here:
In addition, I personally prefer to manage the snapshot situation by a combination of TM, Carbon Copy Cloner, and sufficient hard drive (SSD) storage to make sure I don’t run into problems. This approach might require acquiring Macs with large enough internal drives (i.e., 2 or 4TB vice 256 or 512GB, for example).