The issue is that you’re only selecting one side at a time, and you’re trying to assign it to one side of a stereo channel. This is how you’d normally patch inputs, but with the stereo channels it wants you to do both sides at once.
If you select port 47, select “input” and scroll to “start” 47L you’ll have the error, now go to the “end” box and turn it up to 48R and apply will work. You can also do this for patching lots of consecutive normal channels at once.
You can also patch inputs by selecting an input channel, going to the preamp tab and choosing your socket from there.
Chris