This must be a Drive 500, not an ET500 which is an earlier model
There are 4 things to do
Do a general reset (see page 29 of the installer book)
This is under the Advanced programming section, press the > 8 times
Power it down and then back up
Then re-learn the limits
Go to the Quick Learn for remotes and assign them
If it still shows the same error, take it to your nearest ET repair workshop, that is ET themselves, not a sub-dealer
Just unplug the board, not the whole motor
When you take off the board holder, check there are no ants or geckos inside