Hello, and I’m so sorry that this is happening.
The plugin only stores one WordPress option in the database. That option is called styled_calendar_api_key. If you removed the plugin via FTP, you would just need to delete that option to clean up your database.
We fixed a plugin deletion issue in version 1.0.8, so you may need to update before deleting to resolve this problem: https://projectdmc.org/support/topic/cant-delete-plugin-29/
If you’re already using the current plugin version, I would love to find the root cause of this issue in the plugin’s code. I don’t quite understand what you mean when you say:
it says “https://projectdmc.org/plugins/styled-calendar-customized-google-calendars/”
If you’re open to it, would you please share screenshots of the error to clarify?