I recently had my Rrs autobiography in a garage for some work. When I came out it said low battery start engine on the dash.
Didn’t think much of it. I then drove it for 2hrs home.
The next day the same message came up on the dash and has been on for 3 days since.
I bought a new aux battery but was surprised when I removed the cover and it does not have one.
The battery hold voltage of 12.6 when it’s not running, and is 14.5v when it’s running.

Any ideas or suggestions??

Your 2019 model I think uses a capacitor instead of the auxiliary battery. My 2017 has the capacitor so they must have changed the system prior to that. Regards Barry. 2017 Range Rover Sport TDV6 L494 SE Fuji White
2011 Range Rover Sport SDV6 L320 SE Fuji White Loved but GONE
2010 TRAKKA Motorhome Fiat Ducato GONE (What a dud, don't buy one)
2006 Discovery 3 SE TDV6 Automatic Chawton White/Ebony. Loved but GONE
2006 BMW X5 diesel White/Charcoal, Bought new, Gone after 12 years! Was Excellent.
Ex Member Range Rover Club NSW

I had low battery message also. LR said drive it more. Took it to an Auto Electrician who had it for a few days, but found a parasitic drain. Solved.
You could try putting it on a Ctek charger, if no change, take it to an Auto Electrician, not the main dealer Range Rover Sport (gone): Mercedes S63 AMG Coupe: 992 Porsche Carrera 4S: Range Rover Fifty P400e