I created a repository with the exact problem we have in context of using lerna and yarn workspaces here:

Inside the repository I just copied an ext boilerplate example from here https://github.com/sencha/ext-react/...pt-boilerplate
This is the error we get:

This is a debug view

From what I see the extPath, determined by process.cwd() is not good.
It wants to find node_modules in the package folder, but it's actually at the root of the ext-monorepo.
This works fine with "ext-reactor" as there we can specify {sdk: "../../node_modules/@sencha/ext"}
I also logged the issue here: https://github.com/sencha/ext-react/...ment-498273406