My remote key will not lock the car
Have you replaced the battery in your key fob? Low battery in key fob can cause some functions of the fob not to operate properly.
I am 99% you will just need a new battery if that does not fix the problem then you will need to go to a BMW dealership and have your remote reprogrammed.
Search BMW X5 Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
BMW X5 Experts