Can I make TOC conditional and appear only after the user completes the course?

This means without any doubt that you are not using a playbar but custom navigation buttons, correct?

How is the course set up: is the navigation that way that the learners have to view each slide, so that when they end up on the last slide, the course has been viewed? If that is the case, create a user variable v_visit (Boolean) with a default value of 0. Assign the value 1 to this variable On Enter for the last slide.

You will have to activate the TOC in overlay but make the default Expand button (>>) invisible by replacing it by a transparent PNG in the TOC settings. You will also need a custom toggle button to open/close the TOC. Have a look at:

1 action = 5 Toggle Buttons - Captivate blog

Put this toggle button for TOC (will label it Bt_TOC) on the first slide, time it for the rest of the project, always on top but make it invisible in output. Check my latest blog post:

1-2-3 Eye Buttons Mystery - Captivate blog

If you have an interactive object on the last slide, you can add the command 'Show Bt_TOC ' followed by 'Enable Bt_TOC'. From that moment on the TOC will be available to the user. You can also check the option 'Navigate visited slides only ' in the TOC settings. In that situation you don't really need the tracking variable v_visit.

If you don't have an interactive object to make the button visible, create a conditional advanced action to be triggered On Enter for each slide:

  IF v_visit is equal to 1

      Show Bt_TOC

      Enable Bt_TOC