In Storyline, how can I prevent the Player Next button from working until all layers are visited?

One thing I'd add to Judy's answer: locking Next doesn't lock the other exits. The player menu is the big one — if your menu lists slides, learners can click ahead and skip the gated slide entirely. Set the menu to Restricted in Player settings, or turn it off for a fully linear course. Same with the seekbar: scrubbing the base timeline never opens your layers, so if your player shows one, treat it as another bypass route to test.

My hand-test checklist for this pattern, in the published output (not just preview):

1. Visit all four layers in any order → Next enables.

2. Return to the slide → Next is still enabled (this is the one that catches most builds — the conditional trigger Judy described handles it).

3. Try the menu and the seekbar → no bypass.

If step 3 fails your requirements, that's when the dialog-layer version Judy mentioned earns its keep — dialog layers block the player chrome entirely.