Some of the endless possibilities:

• An invisible UI element (such as an image with alpha set to 0) is covering the pause menu and blocking raycasts
• You have a canvas group in the pause menu hierarchy somewhere and accidentally unchecked "interactable" or "blocks raycasts"
• The canvas is missing a Graphic Raycaster
• "Raycast target" is unchecked on the button UI components in the Inspector
• You're getting some error that prevents the UI from working. Check the Console for errors (the Console is not visible in the video).

It looks like your code is working, since the cursor gets unlocked when the pause menu appears.

Some of the endless possibilities:

• You accidentally deleted the GameObject with the Event System component (as noted by DMGregory, this is not visible in the scene hierarchy in the second half of the video)
• An invisible UI element (such as an image with alpha set to 0) is covering the pause menu and blocking raycasts
• You have a canvas group in the pause menu hierarchy somewhere and accidentally unchecked "interactable" or "blocks raycasts"
• The canvas is missing a Graphic Raycaster
• "Raycast target" is unchecked on the button UI components in the Inspector
• You're getting some error that prevents the UI from working. Check the Console for errors (the Console is not visible in the video).

It looks like your code is working, since the cursor gets unlocked when the pause menu appears.