Sorry I missed that you posted up your specific errors here!
Okay so this is basically saying that the cross-env library is not being loaded.
To start troubleshooting this let’s
1. Make sure that the cross-env package is listed in your package.json file. It should be by default.
2. Manually delete the node_modules folder in the jsforwp-blocks folder and try running npm install again.
3. This should make sure that cross-env is loaded.
Let me know the results and we’ll go from there..