For any reason if you're not able to create an API Page/Query for a system table, you can try for the below workaround
1. AL Developer to create a global Function inside BC, which will extract the contents of the notes into a text field in Sales Header Table. The AL developer can use 2 text fields with 1024 characters each, if you think the notes will be containing good amount of data. It is not suggested to go beyond 2 fields as it will just create clutter. Whatever data after 2048 characters can be truncated (if this works)
2. This function will be triggered by the API or can be called manually at any point. Once this function is called, the notes data will now be populated into your 2 fields
3. Your API can now access the Sales Header table fields easily
