I have a misdiagnosis on my medical record can i join the military?

If the misdiagnosed: Condition is disqualifying for enlistment/ conditioning - then it would be need to be well clarified before hand with a waiver request. If the issues is just that you don't agree with diagnosis - then that will not get your waivered.

Yes, but you're >40y: So that's going to be tricky. As long as you can get a documented persuasive evidence-based medical opinion that rebuts the misdiagnosis, the MEPS CMOs have the full authority to clear you for enlistment. Just make sure you bring the relevant documentation - both the misdiagnosis documentation, and the second opinion. I've done that for applicants. Others will be able to as well. Case-specific.