You can still join. Honestly, though...are you sure you can perform the training? I don't mean to scare you but sickle cell trait is supposedly one of the prominent causes of death among recruits, along with undiagnosed heart problems and dehydration. Every so often a story comes up in the news about a recruit that dies during training, and those are always the first things the doctors look for. Maybe you should talk to your doctor and make sure you are able to handle the physical stress.