I facing an issue with modemmanager during the compilation of OpenWrt 19.07. I have no idea where the problem comes from.
When I update the feeds and install them, I get that warning:
The issue is just cosmetic, a warning, unless you really try to compile that modemmanager package.
And the warning only surfaces, if you have an un-updated feeds repo, like from 19.07.0. if you have properly updated the LuCI feed to the head of the 19.07 branch, you will not see that error.