How to dual boot an iMac 2020, 27", T2 from (Catalina & Mojave)
Solution 1:
Your iMac shipped with Catalina. Generally, Apple does not support using an OS older than what the device originally shipped with. Yes, there are the occasional exceptions. Unfortunately, your model is not one of them.
If you really need to run Mojave on your system, then you should probably consider running it within a virtual machine.