Missing from your description is the routing of the audio — the MODX has ten USB bus outputs.

Your question is mostly a Logic Pro question. (I’m not a Logic Pro user but what you need to do is fundamental routing). You need to to know which of the MODX bus outputs you are using. If you do not change this from the default MODX setting (this will be “Main L/R” — the first two listed of the ten possible buses arriving in your software from the MODX). In other words, somewhere in Logic are the 10 audio buses coming from your MODX - You must select them and route them... accordingly).

In the MODX, each Part can be routed to Outputs: select the Part > press “Edit” > “Part Settings” > “Part Output”

The MODX has 10 possible Outputs:
Main L/R
USB 1/2
USB 3/4
USB 5/6
USB 7/8

(The USB numbered pairs are also available as individual mono buses (as well)).
In Logic these may simply be listed simply by their numbers, USB 1-10 (but please recognize that the first two are the “Main L/R”.
FYI - the “Main L/R” Outputs differ from the other Stereo pairs in that the “Main L/R” can include the System Reverb and Variation Effects and the Master Effects.

In Logic you must setup an appropriate Stereo Input that is set to receive these outputs.
Then you must set your audio track to use the Stereo Input you just created.

Refer to your Logic Pro Manual for details about setting up audio inputs.

Extra Credit:
While it is possible to simultaneously record MIDI and Audio, it begs the question: Why?
Most workflows go, record MIDI first... make corrections or edits to your resulting data... then in a separate operation playback the corrected MIDI data to the Tone Generator (MODX) and have it *return* the audio to the DAW — which you capture with an Audio Track set to receive the signal from the hardware synth on the appropriate buses.

The reason this is preferred is simple, if you make a mistake and need to correct it, it is easier to correct it as MIDI data. Recorded simultaneously, you’d still need to do both over again. Not saying you shouldn’t record both MIDI and Audio simultaneously, just questioning why you would want to...

Thank you Bad Mister for your swift reply. I've been trying all possible combinations with routing the audio. There are two videos on Youtube which go through the whole procedure in detail but these are all relevant to the previous version of Mac OS, even the driver on Yamaha's website mentions compatibility with Mac OS versions 10.12 to 10.15 and I am using 11.1. I am suspecting that Yamaha's MODX driver for Mac is outdated and the fact that my Roland HPD-20 works perfectly well with Logic adds to my suspicion. It seems that the tone generator on MODX is not able to return the audio to the DAW. I am wondering if anyone else who is using Logic on MacOS11.1 with MODX has similar problems.

RE: Extra Credit
I agree with you on the workflow procedure. I've mentioned the simultaneous recording to show that there is no problem recording MIDI data from MODX.

I agree with you on the workflow procedure. I've mentioned the simultaneous recording to show that there is no problem recording MIDI data from MODX.

Ok good, that made you sound like you didn’t know what you were doing...
...not even Apple can break MIDI (lol).

I’m not familiar with the Roland device you mention — does it have a built-in 10-in/4-out audio interface, like the MODX? Likely it’s just a simple stereo output

If you have Big Sur, you are probably not going to be doing advanced audio stuff until drivers become available. There always is a gap with Apple stuff when they change the OS (most experienced users know better than to upgrade immediately). But sometimes it happens you buy a new Mac and you wind up waiting for this device and that plug-in, etc., get updated to work with the new OS. Hopefully, the Big Sur compatible/tested drivers will arrive soon enough. Make note of this gap between Mac OS updates and driver compatibility, if only for future reference. (Many learn a difficult lesson if they update their computer mid project - which is not recommended). It can be weeks or sometimes months depending on what they’ve changed... Apparently, this is a major change in chip... so who knows?

The MODX may be able to run as a Class compliant device until the official drivers arrive (unless they’ve changed this too), until then you’ll either need to be patient or run a different OS.

Extra Credit: check out these articles on the subject...
Link: Additional reading of interest mileage varies greatly from company to company. A company the size of Yamaha has a ton of devices to test and verify... please be patient.

Link: Should I Upgrade to Big Sur?

Thank you very much for your detailed and thorough response and for the links to additional info. I'll be waiting for the new driver release.