@Kevin-Monahan sorry to tag you again.
I found Shortcut Key M does not work on the Adjustment Layer in the Source Panel . Clicking on the add Maker icon with the mouse works once, and then it never works again.
The same thing happens to Bars and Tone and Black Video in Source Panel.
Whether in the latest version of Premiere Pro 23.4 or 2023 and 2020
But the general video and sound is no problem.