I know why you are here, you had the same problem I had and it is taking you a long time to find a solution.

You will need to delete all of the files in your Stellaris folder, however uninstalling through Steam will not work.

Go into your Stellaris folder and delete everything manually and reinstall the game. Previous backups will not work since the files that need to be removed are still there. Now you should be able to get achievements again.

If this did not work please look at this link or contact Paradox staff -> https://forum.paradoxplaza.com/forum/threads/stellaris-achievements-locked-due-to-modified-checksum-despite-having-no-mods.1358035/