There is free disk space (df shows 90% free), but you cannot create a file. What is the reason?

If the df command shows that 90% of disk space is free, but you encounter an error when trying to create a file, possible reasons could be:

• Inodes are exhausted. Even if there is free space on the disk, the filesystem is limited by the number of inodes — structures that store information about files. If inodes are exhausted, new files cannot be created.
• User or group level restrictions. For example, disk quotas on space or number of files.
• Permissions. The user may not have rights to create files in this directory.
• The filesystem is mounted in "read-only" mode due to errors or settings.

You can check the number of free inodes with the command:

df -i

If inodes are exhausted, you need to delete unnecessary files or increase the partition size. Also, check quotas and permissions.