If you cast Pact of Negation and you don't have the mana to pay the "you lose" tax, BUT you do have an activated ability that will win you the game during your upkeep, Arena will by default not allow you to use the activated ability unless you lock in priority control on your opponent's turn.

For example, you play Goblin Charbelcher. Your opponent casts an artifact removal spell on their turn, you counter it with Pact of Negation. You have 4 red lands in play. The turn passes to you, and you should be able to activate Charbelcher during your upkeep because the Pact of Negation "you lose" ability is on the stack to respond to. But instead, Arena's UI will cheat you out of a win.

In order to make it work, you have to CTRL, then Shift + CTRL before you cast Pact of Negation to lock in full priority control, so you have a chance to use the activated ability in response.

The UI by default should allow you to respond to anything on the stack if you have an instant speed action that can be used, regardless of what phase it is.