Can't start Lightsail server under root account

Hello.

This may be the case as described in this document.
We recommend that you contact AWS Support by opening a case with "Account and billing".
https://lightsail.aws.amazon.com/ls/docs/en_us/articles/create-policy-that-grants-access-to-amazon-lightsail

If you continue to get the 403 error after trying the above steps, contact AWS Customer Support. In some rare cases for AWS accounts created before 2011, support will have to manually subscribe your account to Lightsail.