I've hit the same wall this week - every reel or video I try to boost just refuses to go through. Even ones I had running a few days ago. No error message that actually explains anything, just a dead end.
After digging through the usual support channels and a few closed groups, here's what I've pieced together. Meta seems to silently flag certain reels for eligibility - could be music rights, copyright claims, or even engagement bait triggers. The flag doesn't show up in any obvious dashboard. Sometimes it's temporary - the Page or IG account loses advertising permissions for a few hours. Other times, the new boost flow inside the app is just buggy.
The workaround I've started using: recreate the ad manually in Ads Manager instead of the Boost button. Build it as a regular video ad with the same creative. Takes a few extra minutes but bypasses the unstable boost feature completely. Honestly, relying on Boost has become a liability for any consistent content repurposing workflow.