If you get this error:

The attachments upload directory is not writable. Your attachment or avatar cannot be saved.

Assuming you already checked the permissions and they are set properly, another cause for this issue is if you created your forum locally then uploaded it to your host.

There's a few settings inside the datatbase which point to paths. You will need to go on the live server into the database and ensure the paths reflect the live server. The config php file is not enough.

The table in particular you want to look at is smf_settings. Just go through the entries and wherever you see a path, update it to fit the proper path.