Can you fail your first probation drug test?

No. If you think you will fail, the officer wily usually ask if you will pass or not and tell the truthl. Unless there is a legal circumstance that you couldn't have been doing drugs before your initial probation test , you wont get in trouble. Usually they'll send you to counseling/rehab. But don't worry if the charge if for drugs, then they'll understand. Just do not fail it again.