''Slip'', in the context above, means the same as ''to make a mistake or error'' / ''to fail to do something''. As a matter of fact, the phrasal verb ''slip up'' is the correct idiom used to convey this meaning. However, most people choose to say only 'slip' in place of 'slip up'.
Here are some other examples that may help you:
E.g.: ''I think your brother slipped (up) on compiling this list - there are a lot of names missing!''
E.g.: ''I slipped (up) when I said my plane would arrive at 6:00 am — I meant 6:00 pm.''
E.g.: ''She slipped (up) when she delivered those invitations to the wrong people.''
Hope that helps!!!