Young Buck in trouble with the law because he didn't pee on time

Young Buck may have a little performance anxiety ... he's missed two court-ordered urine tests and has now been warned ... third time will NOT be a charm.

Buck -- who's on probation for gun possession --  has had a steady stream of drug tests since November when he got out of the slammer -- all with negative results.  But he bailed on the 2 most recent tests. 

Buck admitted in court docs he screwed up ... but his probation officer was pissed off and threatened to throw him in jail. 

Gee whiz.