When you get "You cannot delete default website My Website...", you must switch to a different website as your new default first and only after that attempt to delete the website.
This is how to do it:
- Activate the Developer Mode in Settings
- Then in Settings in the top menu go to Technical -> External Identifiers
- In the search box enter 'default_website' and click "Search External Identifier for: dafault_website"
- in search results you will see "website.default_website" in the "Complete ID" column, with Display Name showing the name of the website you are unable to delete
- click on that row
- in "Record ID" field enter the ID of your other website that you will be making a new default (if you don't know the ID, try entering 2 or 3 or 4, until you see the name of the desired website in the "Display Name" field)
- That's all, you can now go back to the list of the websites and delete the former default one.