Why does WordPress say it can't create a directory when my folders are already set to 755?

755 only describes the permissions; it doesn’t prove the PHP process that runs WordPress owns the folders or belongs to a group that can write there. WordPress checks whether it can create files with matching ownership, and it may fall back to FTP or SSH when direct writes aren’t safe, so don’t try to fix this with 777. Check the owner and group on `wp-content`, `uploads`, `upgrade`, and their parent folders, then check your account’s free disk space and inode/quota limit. If this started after a PHP, web-server, or host change, send your host the exact error path and ask which user PHP runs as and whether it can write to that directory.