If you do “Get Info” on the disk in Finder, does it say that any of the available space is “purgeable”? How much?
Is it your internal (system) disk or an external? If it’s an external, please check “Get Info” for the internal too – how much available and how much purgeable space does the internal disk have?
Okay, yes, 61 GB “löschbar” is purgeable space, so it’s not really free. A future version of the installer will be able to handle it better, but now you’ll need to decrease the purgeable space before installing.
There is no easy way to do that, except maybe with some 3rd-party program, but it can be done by (almost) filling up the disk with large files that you then delete. When you have added 35 GB of such “dummy files,” the purgeable space will begin to go down when you add more files. Let’s say you add 115 GB of files (so you have 5 GB available) and then delete them, you will have at least 115 GB truly free space, and then you can install the library.
To add such files, it unfortunately does not work to duplicate files in Finder (assuming the disk is in APFS format), because such copies do not take up any additional space. You can instead copy large files from a different volume, for example from a mounted dmg. Or if you know how to copy files using Terminal commands, you can copy/duplicate files within the disk.
