Why might a situation occur where there is free disk space on Linux, but adding new files is still impossible?

This situation can occur because the available inodes in the filesystem have run out — these are structures that store metadata about files.

Even if there is free space on the disk, if inodes are exhausted, it is impossible to create new files.

You can check inode usage with the command:

df -i

If the number of free inodes is zero, you need to delete unnecessary small files or increase the size of the filesystem.

Another reason could be user quota restrictions or access rights issues, but most often — it is the exhaustion of inodes.