my diplomacy is low so i want t max it yet my missions keep getting foiled i got 24 troops, 12 bulwark battle cruiser on it as well o

Park the deathstar there... they will F-ing love you then mate

rebel side m8

are there any other planets in the system you can win a battle at?

I'm not clear on the circumstances here. Is this system controlled by you but loyal to the enemy, or is it neutral? (You can't do Diplomacy missions on enemy-controlled planets; use "Incite Uprising" instead.)

The only way a Diplomacy mission can be "foiled" is if you send it to a system with an enemy fleet in it. However, it can "fail" if you send it to a friendly/neutral system that switches to the enemy while the mission is underway.

Also, Diplomacy doesn't work all the time. The success chance depends on the total Diplomacy score of the character(s) on the mission and the current level of support. (The maximum chance of success is 70%.) If it succeeds, the support bar moves somewhat toward your favour; if not, nothing changes. Your diplomats will keep trying until 1) the system supports you 100%, 2) you abort the mission, or 3) as mentioned above, the system switches to the enemy for other reasons.

One other thing: it is possible for enemy operatives to Abduct/Assassinate your diplomats.

Is the planet in uprising?
Maybe you need to quell that first?