Make sure the Pegasus R4i MPX module has all the drives installed, properly seated and locked in place. See image below with lever on lock position.

 

Also, make sure the Pegasus R4i MPX module is sitting in either one of the two available MPX slots. See image below. Pegasus R4i can be installed in Slot 3 or Slot 1.

 

If R4i MPX module is properly seated make sure the Mac Pro lock key is engaged:

When the system is turning on you will hear a single beep. This will indicate the R4i MPX module is now detected. Boot into the OS and install the Pegasus Utility found in the root of the volume.  The Promise utility will allow you to monitor and manage storage.

 

Once you install the Promise Utility, you should be able to login to view the status.

Front View:

 

For additional tips see the following KB:

https://kb.promise.com/thread/proper-installation-of-pegasus-r4i-inside-of-a-mac-pro-2019-release/