For those none of the above worked, mine is two issues:
1. Too many labels to show, but my laptop is not fast, none discrete GPU. Although the right bottom corner "refresh" icon is stop rotating, the labels still didn't show up.
So I googled and arrived on this web page. After I read through, and return to my ArcGIS Pro 2.8, the labels showed up by itself!
2. A lower layer could cover up an upper layer! Yes, I saw with my own eyes. I re-order the two layers many times. The lower one will cover up the upper one!!!