I try to accept jobs from Jim Stacey, and tells me I need to be a mastermind. I try to advance and it tell me to do more jobs. I finished all the other jobs. What do I do???

Are your Attributes and Skills good enough?

To become a Mastermind you need your Agility and Personality to be at least 34, and you also need one favored Skill at least 80 and two favored Skills at least 30

The favored Skills for the Thieves are Acrobatics, Light Armor, Marksman, Security, Short Blade, and Sneak

You can check all this info in your stats Menu, and by hovering the pointer over your Factions (listed in your Menu also)

Have you done the Bal Molagmer quests he gives you? I think he gives them at a lower rank then his final quests, so you may be able to get those done.

Confirmed that he gives Bal Molagmer quests to lower rank. I've done all the quests from other masters and now doing Bal Molagmer with my gentleman thiefish Wood Eld, although he doesn't let me touch his other quests yet. Requires a skill at 60 which I'm still lacking (to get promoted to Captain).

Apparently your stats are fine to become Mastermind if it really tells you you're only lacking in job department. In that case, I'd also guess you haven't done the Bal Molagmer quests. You should wear those gloves and play a little Robin Hood.