Hello and
. Not the best start, but you’ve found a good Forum and the folks in here will try to help where possible.
First thing you need to do is get the error codes logged in the car checked. Use somewhere that has the correct software (like a BMW Dealership, or a Bosch Autocentre, or a ZF transmission specialist). Generic code readers will often not give you the correct diagnostic codes from which you can confirm what might be the problem.
You haven’t said what age your E53 is? If it is a late 2003 onwards (known as the facelift model) build (see label under bonnet) with a 6-speed box, or a pre that age with a 5-speed. What kind of mileage is on it and has it ever had a gearbox problem in the past (that you know of)? What engine?
It could be anything from low transmission fluid meaning insufficient pressure up to something more serious needing a rebuild of some sort, and anything in between. The correct codes will tell you (and may help members on here to help with their own experiences too).
If there are any odd metallic noises coming from the transmission area the stop driving it at once! Those noises indicate a serious issue which will get worse. You will need recovery if that’s happening. Similarly, any reluctance to go into gear, especially reverse, is a sign the box is in trouble.
Unfortunately the E53 is known to suffer from gearbox issues - as were many other cars fitted with the same units - but that doesn’t mean it’s happened to you too.
Go with simple until you know otherwise. Repairs could be a few quid for a top up with the right oil to around £2k for a rebuilt box. Don’t ask BMW to do it as they will just offer you a factory rebuilt one at really silly money, but they will do a code read for about 30mins of labour charge. BMW will not repair, just offer a replacement.