perhaps a silly question but are you sure that there is no packages.json in the media manager folder

No package.json.

I asked because it happened a few times to me when I was testing an old PR.

I was on slightly older versions than you with no problem.
I just update to latest versions and no problem
node v14.5.0
npm v6.14.5

windows not mac

On Mac (10.15.6 beta) with

node v14.5.0
npm v6.14.5

I cannot replicate your issue - sorry. Only error I have looks like an upstream error.

> fsevents@1.2.13 install /Users/phil/Sites/joomla-core/joomla-cms4/node_modules/watchpack-chokidar2/node_modules/fsevents
> node install.js

  SOLINK_MODULE(target) Release/.node
  CXX(target) Release/obj.target/fse/fsevents.o
In file included from ../fsevents.cc:6:
In file included from ../../../../nan/nan.h:2884:
../../../../nan/nan_typedarray_contents.h:34:43: warning: 'GetContents' is deprecated: Use GetBackingStore. See http://crbug.com/v8/9908. [-Wdeprecated-declarations]
      data   = static_cast<char*>(buffer->GetContents().Data()) + byte_offset;
                                          ^
/Users/phil/Library/Caches/node-gyp/14.5.0/include/node/v8.h:5254:3: note: 'GetContents' has been explicitly marked deprecated here
  V8_DEPRECATE_SOON("Use GetBackingStore. See http://crbug.com/v8/9908.")
  ^
/Users/phil/Library/Caches/node-gyp/14.5.0/include/node/v8config.h:402:39: note: expanded from macro 'V8_DEPRECATE_SOON'
# define V8_DEPRECATE_SOON(message) [[deprecated(message)]]
                                      ^
1 warning generated.
  SOLINK_MODULE(target) Release/fse.node

Nuking the node_modules folder from administrator/components/com_media seems to have fixed it.

doh - sorry - thats what it was - not the packages.json