Home manager doesn't activate on reboot

I recently installed home manager. Amazing software. Only issue is that it doesn’t activate when I reboot my computer and I always have to put home-manager switch every boot. Do I have to put in an autostart script or is there an actual issue with it

See step 4 of the standalone installation instructions: Home Manager Manual

I don’t use flakes. The issue was that I wasn’t using home-manager build

Woops, my bad, I meant this heading, you need to load the home-manager vars in your shell init files: Home Manager Manual

Almost certainly not, home-manager build will not deploy your configuration at all, it just builds it and gives you a result symlink so you can look at the build outputs. Don’t assume that was the fix and use it exclusively from now on, if you do your configuration will never update again.

Any chance you have also enabled shell configuration in home-manager between your last reboot and your last home-manager switch? That would add the correct env loading without you having to manually configure that.