incompatible Disk Warning

I have a Macbook Air 2017 with 256gb of storage. It came pre-installed with macos Mojave, and eventually i updated to Catalina, then Big Sur. However, i didn't like Big Sur and reinstalled Mojave via bootable USB. Now whenever I start my mac, an 'incompatible disk' warning appears. It doesn't appear to have any major effect on my system, however some problems occurred when I tried to install Windows 10 via Bootcamp. It's saying I don't have any storage free when in fact, i have about 150gb free according to Disk Utility. Is there anything I can do?