I think Frappe Cloud restricts development mode. You need SSH access to properly export all kinds of customisation and then push the App to a git repo. For me local development and exporting worked. Then I pull my git changes to cloud.
Please do correct me if I am wrong.
Yeah, as I understand it (still new at this), Frappe Cloud is designed with a certain level of security and support in mind and won’t allow the full access necessary to create a custom app. You can install already-created custom apps, and if you have the right plan, you can create server scripts, but you can’t create a custom app on Frappe Cloud. You need to set up a development environment elsewhere, activate developer mode, and then you can create a custom app. Once it’s working, you can install it on Frappe Cloud or elsewhere.
(I’ve installed ERPNext on a virtual private server using the frappe_docker project, but you could simply do a local install if you expect to be the only developer working on your custom app.)