The mod Oxidized Fabric 1.20.1, is installed in both, my server and my client, however the server doesn't allow us to log in, keeps sending the error that the mod is mismatching. Please help

yes, both have the same name, it's weird because the mod has not been updated an I was able to join the server before

That means your server and client do not have the same version of Oxidized. Check that the file name is exactly the same version.