SO I have a 2nd PC with the monitor stood up portrait to save space, now other pinball games like PFX and the new version of Pro Pinball are fine but I keep getting error messages about non-flip and cant run in this resolution.

Is there a setting somewhere I can change?

in some windowed modes a method might be instead of 640x480 try 480x640

also on Linux for whatever reason the Displays I have rotated or flipped up sidedown the default setting on this seems to be 90.

For Windows 11 found a display orientation option but it is greyed out. WindowsKey+I to get to Display. May need to be administrator. One site said Ctrl+Alt+ Arrow Keys

I know this is an old post (a year old) but maybe it can help someone. There IS a way to turn a "modified portrait" mode. Look for a mod named "PinCabView" (now in version 4.3) The idea of this mod is to make it work in a full portrait mode for use on a virtuall pinball table. If I'm not mistaken, you can even send DMD to other screens.

It works for every Pro Pinball table on GOG and also for the web, as well as other pinball games. Check it out, maybe it'll help with what you want.