Hello there @chiblue3d!
Since UE 5.4 and 5.5 are working as intended, the error you are receiving aims at a compatibility issue between UE5.6 and the MSVC toolchain.
Now, just to make sure this is not simply corruption due engine migration, the first step would be a cache clear. Please close UE, go to your project directory, and delete folders Binaries/, Intermediate/, Saved/, and .vs/, then regenerate your project files from VS, and re-open UE.
The next step, is to make sure that your UE and VS setup is properly configured, and that all installed elements have the correct version. You can review this process with the documentation below:
Finally, in Rider, open Settings > Toolchains, and manually set the paths for your MSVC and SDK. After that, try to compile again. Hope this helps!