Ding, ding, ding, ding, ding!
“Unable to create directory” is most often caused by inodes in my experience. Directories consume pretty much no disk space, so file size limits or storage limits are not the issue. However, directories do consume an inode
And if you check the control panel, you can see that your inode usage is at 99%, which is so close to the limit that it will probably cause problems.
Here you can read more about inodes:
I took a look at your account and I see you have two copies of your website, one in the htdocs folder and one in the aletrisdigital.epizy.com/htdocs. Your domain is linked to the latter folder, so you can safely remove the main htdocs folder and free up a lot of inodes on your account without having to make any changes to your site.