Sometimes, it happens that the achievements for completing all Covert Operations, Counterstrike, Aftermath, or even the regular campaign missions (including any bonus gallery unlocks) don't unlock correctly, despite the necessary conditions having been met. Here's what to do to remedy this issue.

First, navigate to \Steam\userdata\<your ID>\1213210\remote\. Depending on which game is affected (Tiberian Dawn or Red Alert), remove Player_TD_settings_1.bin or Player_RA_settings_1.bin. Second, make sure cloud save syncing is disabled before proceeding any further.

Now, start the game. All your saved games should still be there (provided you did save), so the fastest way to unlock those achievements is to simply load random saved missions and finish them. The easier the mission and the closer you are to finishing it, the better. Finish as many missions as needed to unlock the achievement (check beforehand how many missions are missing) and the achievements will be unlocked at last. If you don't have any saved games ready, just play whatever mission you want from scratch. Of course, if you want to unlock the Aftermath achievement, for instance, it should be Aftermath missions.

Once all achievements have been unlocked, quit the game, copy your old Player_TD_settings_1.bin or Player_RA_settings_1.bin back into the folder mentioned above, overwrite the existing file (which has been generated for the previous session), enable cloud save syncing again, and start the game. That's it.

In case you run into any issues, please let me know in the comments below.