What is XCode version that you using to build iOS app?
I simply exported from C3 as an xCode project.
2. Unzipped it and opened the .workplace file in xCode
3. Dragged over my icons from my old project.
4. Made sure all the settings were right and the version code was later than the last one.
5. Tested it in the simulator and my personal device.
6. In the Menu went to: Product -> Archive (I actually had to restart xCode because it was throwing up an error when I tried to export it, but after I restarted xCode it worked. I found that out by going to stackoverflow and everyone was having the same problem with xCode. Apparently a restart fixes it. I also had to turn off automatic-signing then on again to fix that, lol. XCode is really buggy. )
7. Then I uploaded it using xCode to my appstoreconnect website profile and it worked!
Only took me about 3 hours, but worth it in the end for sure. Now I can do it easily since I've been through the whole process a few times.:)