Only 1 attempt in TB missions

You don't lose anything, if you've got the right teams modded right, platoons done etc then RNG is irrelevant. Even then if you lose the character GP still gets added to the score, so its not even a case of risking not being able to deploy that GP.

From CGs point of view multiple attempts are not allowed as it removes the incentive to mod and zeta characters to ensure the 1 shot, rather than clear missions with non-optimal levels of character.