Hi I can't get both the numberplate lights to work at the same time with led bulbs
I can get one to work but it must have a normal buld in the opposite bulb holder am I doing something wrong

Try the one that's working in the other side first then try reversing the polarity of the none working bulb? It's also quite possible that one LED is no good and you can test that in the working side as well. Also, check the metal contacts on the receptacles carefully without touching both sides?

I've tried every combination and they only work with one normal bulb in the other holder I'm thinking it must be a led bulb issue they all work but I have to have a incandescent bulb in the other holder

Are the LED bulbs you have "canbus compatible"? Normal LED bulbs only draw a fraction of the current drawn by normal bulbs, maybe the output doesn't work properly when the current is too low. Canbus compatible LED bulbs contain extra load resistors so they draw more current.

On ebay and/or aliexpress you can also find complete replacement licence plate light with multiple LED's build in.

What Grumsel said
I had LED bulbs in, worked OK but always showed a fault now replace with loaded ones
from Alliexpress and all hunky dory now.
Dave DAVE.

New bulbs it is then
Cheers James