A break needs at least 15 minutes left in the block. With less time than that, we don't start the break. Slow Time Club tells you time's almost up, or, if you tapped TAKE A BREAK on the block screen, you get a No break this time notification.
Why
Apple's Screen Time only lets apps schedule things at least 15 minutes ahead. When you take a break, we use that to make sure your block comes back afterwards. With less than 15 minutes left, the block might not come back, so we don't start the break. We'd rather say no than leave your apps unlocked by mistake.
What you can do
Next time, take your break earlier, while there are more than 15 minutes to go.
Or wait. The block ends by itself soon.
If it's a Soft block and you're done, end it instead. Tap STOP on a Quick Block, or End Session on a Focus Session.
This is also why a Quick Block or Focus Session must be at least 15 minutes long.