PE2023 Render issue / Black Bars

The tech specs for PRE 2023 require MacOS 11 or later. 

Delete your media cache and preview files and export again. See how that goes.

I'm not sure I fully understand your problem.  If you render out a portrait-oriented 4k video and view it on a widescreen (16:9 aspect ratio) monitor, you will see black bars on the side, even if the project settings fit the original video size.  So, if this is not the problem you are describing, you need to

• verify that the project settings do indeed match your portrait oriented video (if not, start a new project and make sure you have the correct settings - Portrait 9:16 2160x3840)
• when you export the file, make sure you choose the correct format for your desired display.  If you intend to output to a phone or tablet for display in portrait view, use the Devices tab for the Portrait display.

I'm afraid I don't understand what you are trying to convey in your screenshot.  Is this a comparison of the original video and the rendered video displayed on a phone?  If so, then it seems to me that either your project or your export settings are wrong.

Since this is the Photoshop Elements forum (and I know almost nothing about Macs), I am going to move your post to the Premiere Elements forum where you will get better help than I can give.

Additional note: When I click the center button on the footage to "stretch" it goes back to the correct uncompressed format, but then it needs to be rendered again.  So it's a vicious circle. My choices are either unrendered footage that stutters or rendered footage that compressed.