I've had some struggles with this issue. Just got a new Samsung S21. Tried to open the program. It starts and then says unknown Amazon app store error. Re-download the app.
Well, the app isn't listed directly in the Appstore. It was still listed in My Apps but it will not open. Contacted customer service. I found my online order details. The customer service rep asked me to re-download the app. I told her it was no longer visible within app store. This was a concept she could not grasp. She had me then uninstall the app store app. Told me to re-download it from Google playstore. Well, that's not possible cause Amazon Appstore is no longer listed in Google Playstore. She couldn't understand that.
She had me try another device. My Asus Zenpad. Uninstalled app store. Same deal. Appstore app not available on google Playstore.
She put me on hold for a few minutes and came back to say my devices were not compatible.
While waiting for her I made some progress. Did a search on Google for 'Amazon appstore missing from Playstore'. Found a post saying it was not available at the Playstore but had a link to download the app from Amazon.com. Turns out you just search on Amazon.com and the Appstore download link shows up.That worked and the appstore installed fine on my tablet. IReal Pro was still listed in my programs. Clicked on install and I was back in business on the tablet.
Onto my new phone. Downloaded the appstore app directly from Amazon.com to my new S21. Installed fine. Unfortunately, iRealPro is not listed as one of my programs on my new phone.
So, halfway there.