Can a unit prevent a 25B soldier in the primary promotion zone from attending the board if they have not obtained Sec+?

Sec+ is not a promotion requirement. They can however say most anything else to keep you from going forward. So while technically no the answer is yes.

I was promotable when I got out in 2012. From what I remember, the Sec+ certification was not a requirement. If so, then I would have been the only 25B in the battalion that could have gone to the board.

That said, look at points. I went to a 40 hour "boot camp" course prepairing for the certification. I was given a certificate of training signed by an O6. So the class gave me 40 hours of Military education. Then the certification itself gave me 10 semester hours of civilian education. It's all promotion points, so get em if you can. Also looks good on the NCOER, especially when those above you don't have it.

That is a negative, I was in the same shoes. Although it is a great thing to have per Advance Leader Ship School House requirements , you do not have to have it the Reg does not states that it is a mandatory requirement. I do recommend getting it anyways, it will set you apart from your peers.