I’m not an expert on this, but I would look at the directory permissions through FTP.
In your FTP client, in the WordPress directory, navigate to: wp-content and the themes directory in it.
…and check what the permissions are set to on both. I believe that both the wp-contnet and themes directories should be set to 755 and owned by your user. If they aren’t, set them to 755, and see if that resolves the issue.
More technical info here: https://codex.wordpress.org/Changing_File_Permissions