So you just need an AP and that's it. I'm saying this because I didn't understand why you were using AP+client mode if you only wanted one AP.
In my case there is no problem to choose the wifi user to which I want to apply the AP task, but... I preferred to use as AP a mobile phone so all the devices connected to it -the Rpi, computers, tablets etc.- have access to the Internet and to the Rpi data too.
What I would do is unplug the external wifi unit and go to openplotter-network. and put it exclusively in client mode:
Make sure you choose the mode, press the apply button and the next button too. After restarting Check that you don't have an AP point. Then plug the external wifi and open openplotter-network again. Try again to choose usb for AP, apply button 1, fill in the values and apply button 2.
After the reboot, it should be working. If it doesn't, the problem could be the wifi itself for some unknown reason. If you have an android phone it might be easier to do the same as me and use it as an AP point.