At what point is it advisable to add a database using Lightsail

Hi,

A database can allow you to scale up easier to multiple instances and store the information wordpress needs to a centralized location. This can also improve your website's resiliency when combined with a load balancer.

You can use the below doc to set up a database if interested. https://lightsail.aws.amazon.com/ls/docs/en_us/articles/amazon-lightsail-connect-wordpress-to-mysql-managed-database

However, to diagnose the above error, you will like need to look at the WordPress error log on the host.

https://wordpress.org/support/topic/critical-error-on-this-website-21/