Opponent cast Mystic Confluence, bouncing my only legendary creature and also countering a spell unless I paid mana twice. I had a Mox Amber in play, which I wanted to tap for mana so that I could replay the creature after my spell was countered. The game WOULD NOT LET ME DO THAT.

The only options were "pay" and "cancel." The option to tap for mana and then not pay was simply unavailable. Nothing I could do would make that selection, despite it being the literally only logical choice. Eventually it timed me out, with my Mox Amber untapped, the creature stuck in my hand, and basically time walked by my opponent. A spell that gives a choice to spend mana should not freeze the player out of every other mana ability for any other use.