Can you be barred if you are not ready for the SGT promotion board?

While I don't agree, and believe it to be an intimidation tactic, I also don't know the Soldier and everything surrounding their situation. However, for the sake of knowledge, I've attached a screenshot from AR 601-280, pertaining to Bar to Reenlistment. Read Chapter 8, but particularly the last sentence of Paragraph 8-4a, as well as all of 8-4d(1) through 8-4d(19)(f). Commanders CAN Bar a Soldier for basically anything, with a few exceptions.

While the 1SG isn't the commander, they are the Commander's senior enlisted advisor, and can make strong recommendations for action.

I say all of this to say, do an HONEST assessment of your Soldier, and have him do the same. If it was truly an unwarranted threat against a documented average to stellar Soldier, then pursue IG for clarification. However, if your Soldier is honestly ate up, and you are trying to defend him on the idea of technical "wrongness" by the 1SG, then your time would be better used helping him fix his deficiencies.

That's just my take on it. It's his career, he should do what's best for his future goals.

Hells no they can't BAR someone for not passing a SOM board. They can certainly keep them from going to promotion boards, but not BAR someone. And yes, I would say this sounds like an IG complaint.

"One of my soliders went to a SOM board in hopes of being ready to go to the monthly promotion board. The SM received a No-Go. The 1SG sitting in told them they had 5 days to retrain and attend the secondary SOM board. If SM receives a second No-Go they were threatened with a Bar to reenlist"

See my response to another posted misguided statement to read my opinion.
" as well as declining an EFMP initiated PCS due to Health concerns."
The only way that reads as reasonable is if the SM is due almost immediate reenlistment, or perhaps needed to extend to meet service obligations IOT be eligible for this potential PCS.
If the SM is found unsuitable for reenlistment (barred, under the provisions of AR601-280, reason for justifying a bar para 8, 4d it would make sense to stop or delay the PCS process..... no matter the justification for that PCS, be it personal preference , normal rotation after time on station or a PCS by exception.. If your unsuitable for reenlistment, that means no PCS as well.