Sorry, I think it's not possible to use Terminate inside a Do Until.
The only workaround I can think of for now is to:
1. In your Do Until (you may have to use "Edit in advanced mode" for this) turn your Do Until into your original condition, OR a check for a specific boolean variable (so it checks for whether your original condition OR this other new variable condition is met.). The other condition should simply check if a new variable you define is "true" (no quotes, actually it should be specifically an expression value). You may need to use a Set variable "yourNewVariable" to "false" right before the Do Until so it knows the variable exists, and to make sure it's false as well before going into the Do Until.
2. In the Run after (previous has Failed) branch, use a Set Variable block to make that other Do Until's condition "true" (again, no quotes, actually it should be specifically an expression value). so it gets out of the Do Until.
3. Right after the Do Until, using an If node, do a check for that variable, and if it's true, on the If yes side, have your Terminate succeeded block there.
Besides the above, I am not sure of another way.
It might be a good idea to have this as a suggestion for improvement, to be able to use the Terminate block inside a Do Until.
You may go to
https://ideas.powerapps.com/ to search and upvote similar ideas, and if it's not there, you may post it yourself.
If you don't find any similar ones and created one, let me know so I could upvote it.
