Bottom of the page here:
https://www.mathworks.com/help/supportpkg/usrpembeddedseriesradio/ug/troubleshooting-hardware-software-co-design.html
Solution is to set the board to external mode.
My board has been working in External mode -
I'm now trying to get it out of External mode.
I first had the warning that,
even though the simulation was set in Normal mode,
the parameter ExtMode was set to 'on' when I tried to build.
I manually set_param('<model>', 'ExtMode', 'off') to fix this,
but this error persists.
I believe the code is not recognizing the conversion back to normal mode,
and I am not sure how to handle this.