The sandbox have been provided by keeping the interest of SAP users in mind. You being a user can utilize the sandbox primarily for learning and evaluation purposes ONLY. Kindly note, it should not be used in any productive or commercial manner.
Let me not call it as a limitation but a few restrictions are applicable. These are:
This sandbox is limited to SAP Build Apps and does not provide access to BTP cockpit or SAP Build Process Automation, or SAP Build Work Zone.
Kindly do not store any private or personal information in the sandbox or any application, which you create in the sandbox.
How to Sign-up and Get Started on using SAP Build Apps Sandbox?
Well, there is already a post written with screenshots to access it at this SAP Community page. Hence, I will just share the direct steps to follow: