Thanks for the submission,
Ditto51!
As
SandHawkTime mentioned, there is a chapter select available in the game, which allows you to go to a specific campaign mission to get the objective missed. Since a full playthrough of the game is not required to get back to the level of the missed objective, we will not apply the missable flag.
For
The Knowledge in Command & Conquer: Red Alert 3, purchasing your "army" before capturing the six structures is seen as a prerequisite to the achievement. This use is excluded from the Shop flag, and thus we have not applied it.
If you have any questions, feel free to pass them along!