Riiight... Since CodeLLDB now brings along its own Python, it may happen to be of a different version (3.8), than your system Python (probably still 3.6 or 3.7). Each version maintains its own set of installed packages: you've installed numpy for python3.6, but not for python3.8, thus the error.
I haven't yet figured out how to deal with this properly, but meanwhile try this:
1. Start a debugging session - any debuggee will do, as long as it stops on a breakpoint.
2. In the debug console, enter "script import pip; pip.main(['install', 'matplotlib'])"
After a bit, it should report that matplotlib has been installed.