EU5 App ID number: 3450310 (You can check App ID in "Steam Library->Right Click Game->Properties->Updates)

1: Use Win + R keys and open Run console, write steam://open/console and OK

2: Check the code of the achievement you want to delete from this site. The codes mostly have the same name. There should be an _ between the words.
https://steamdb.info/app/3450310/stats

3: Return to the console in the Steam interface and type this to delete the "500 Mile" achievement. For example;
achievement_clear 3450310 five_hundred_miles

4: Voila, achievement is gone.

5: For console close same as 1 step, also this code restart steam; steam://exit/console

disclaimer: use at your own risk