I received a garnishment reversal letter from the lawyer of the plaintiff saying that I was released from further liability in conjunction with this garnishment and any amounts withheld on or after March 12th shall be returned to me but then 3 weeks later I receive a letter saying they were going to garnish effective immediately if sufficient funds are available I checked and its for the same case number can they reverse it then change their mind? I'm so confused

I suspect that you received a boilerplate first letter, and a clerk punched the wrong button on the computer when doing a large number of mailings.
Unless you know of some reason why your account shouldn't be garnished?
Can they do this? Sure... a court might be sympathetic if you sue (although I don't know the odds), but that will take a lot longer and cost you money, and meanwhile the garnishment will have started.

I'm guessing that the first garnishment was somehow faulty, so they fell back, regrouped, and did the garnishment again.