Hello @builder,
You are probably missing this step:
Checkout dependencies - Appinventor Sources
Checkout dependencies
App Inventor uses Blockly, the web-based visual programming editor from Google, as a core part of its editor. Blockly core is made available to App Inventor as a git submodule. The first time after forking or cloning the repository, you will need to perform the following commands:
$ git submodule update --init
See also here:
https://groups.google.com/g/app-inventor-open-source-dev/c/tJcDxTGgwRc/m/XjT7eE0hBAAJ