why "not enough disk space" while i have more than 2TB

@chdsl is correct, you have only 24.66GB of Free storage space that is immediately useable at the moment.....that is the absolute bare minimum needed for the normal operation of macOS.

Notice how the "Available" space is 3.21GB with 3.18TB listed as Purgeable.

Purgeable is storage that macOS will release at some unknown time in the future. If you do the math, you notice that:

3.21TB  -  3.18TB  =  .03TB  (approximately 30GB)

Available  -  Purgeable  =  Free

It is extremely unfortunate Apple pushes the Available value since it is extremely misleading as you have just discovered.

It is hard to say what is taking up this storage space, but I would bet you will find it within the hidden APFS snapshots for your backups. Even if you delete lots of data, the data will remain behind if any APFS snapshots existed prior to deleting the data. Once those APFS backup snapshots have been transferred to external media, then the hidden snapshot will be automatically deleted. However, sometimes a snapshot may remain for longer depending on the backup software used and how it is configured. You can use the instructions in the following Apple article for viewing and deleting APFS snapshots. One important detail is missing in those instructions....you must have the "Data" volume selected before you will see the option "Show APFS Snapshots" on the View menu located on the menu bar.

View APFS snapshots in Disk Utility on Mac - Apple Support

FYI, if you delete a snapshot before it has been transferred to your external backup media, then you may be risking your backups.....that is, if you created/modified a file and accidentally deleted it, that copy may never have been transferred to your external backup media.

Within Disk Utility, click on the "Data" volume on the left pane of Disk Utility. On the right pane just below the storage bar graph you should see the Free space value listed. Unfortunately this it the only place macOS actually lists the Free storage space value. Everywhere else, macOS is listing the "Available" space value which is very misleading.

What is the value of the Free space listed?

It says that your Mac storage is practically full. But, try shutting down your Mac and rebooting it and checking again.