Level Sequence has no tracks

[Max.Chen]

Looks like you have a filter enabled. It’s the red dot next to the search field in the Sequencer tree.

Take a look at this post for more details:

Can't add actors in the sequencer to animate them

Glad that helped. In versions prior to 5.4, if you added a control rig, it would automatically set the filter to Selected Control Rig Controls. Perhaps that’s what happened in your case.