Does anyone know how I should go about fixing this?
I want to use the physics mod but its incompatible with iris.

Solution
Physics Mod Pro's versioning is shit at first glance. Anyways, try some of these versions that have 1.20.4 in the name:
https://www.patreon.com/posts/physics-mod-pro-105510754
https://www.patreon.com/posts/physics-mod-pro-105510754
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