I know there are countless posts with this generic issue, but hopefully I can help someone debug their own issue. Most solutions had to do with making sure the app was compiling against the correct sdk, and configuring Appium correctly to open the .app or .ipa file. I was doing both of these and still couldn’t figure out why my .app file kept crashing when starting up in the Appium simulator.
I had to build a .ipa file to run on my device, and then check my crash log there in order to debug what was happening. In my case, my crash log showed a couple frameworks weren’t being found. I made these frameworks optional in the project settings, which fixed the crash.
Next I made sure to set the Xcode plugin to build against the iphonesimulator sdk and not generate an ipa so that I could use the .app file on Appium. Finally it worked, but the key was to create the ipa file first so that I could take a peek at the crash logs in my Xcode device window. Whew!