I have both a Canon 80d and a Canon RP.
When trying to shoot in low light conditions the 80d auto focuses much better than the mirror less camera.
Should I expect that and if so why.
Thanks for your help
There's something wrong if your 80D is better focusing in low light than the RP. The mirrorless camera has a more sensitive AF system. It's rated to be able to focus two stops lower light than the 80D. That means it can focus with only 1/4 as much light as the 80D needs to be able to focus.... all other things being equal.
As others have already said in their responses, it's entirely possible you've made some incorrect settings.
The lens being used is another big factor affecting AF performance and you didn't mention the lenses being used on the cameras. For example, if you have an f/1.4 lens on the 80D and an f/4 lens on the RP, the DSLR's AF system will be getting 8X more light to work with.
It also may have something to do with the type of autofocus motor in the lens. An STM lens is typically not as fast focusing as a USM lens. Some lenses, such as macro, are also slower to autofocus by design. They use a "long throw" focus design that emphasizes precision over speed, because super shallow depth of field typical of macro and close-up photography requires very accurate focus.
Ambient lighting conditions are another major factor affecting AF performance. It may seem the same, but actually be dimmer light where you are trying to get the RP to focus. Our eyes and brains are "adaptive" to different light conditions, where camera AF systems are not. Or the subject you're trying to focus upon with the RP is low contrast with little detail or otherwise a difficult target for the camera's AF to work with. To compare the cameras very accurately, you'd need to carefully test them under identical conditions shooting the same subject.
Finally, it's possible but unlikely that there's actually some fault with the RP that's keeping it from focusing as well as it should. Again though, this is a pretty remote possibility. It's far more likely an incorrect setup by you or differences in the lenses you're using or some other factor.
Start by studying the AF setup of the RP in the manual and look for online tutorials. Those are the most likely answers to solve your problems.
Edit: CHG_Canon is right.... it would be helpful if you would upload sample images with their EXIF intact. Maybe we can spot one or more incorrect setting that's causing your AF problems.