If the column already exists, it can't be added.
But if you WANT it to exist (and it sounds like you DO since you're trying to install the mod), then you can safely ignore the error/warning.

If whatever method you're using to run those SQL statements actually carries on and attempts all the next SQL statements, then you should be fine. But if that one error causes it to abort, then you may need to manually run all the subsequent statements one by one to get them to take effect.