HTTP 5xx Error when deploying wordpress website using Elastic Beanstalk

Hi All. I am following along with the AWS-provided instructional guide entitled "Deploying a high-availability WordPress website with an external Amazon RDS database to Elastic Beanstalk,". However, I receive errors once I upload and deploy the application.

The three error messages I am receiving in Elastic Beanstalk once I deploy are

1. "100.0 % of the requests are failing with HTTP 5xx."
2. "ELB processes are not healthy on all instances."
3. "ELB health is failing or not available for all instances."

Things I've Tried I've tried updating the security groups of the RDS and the entire Elastic Beanstalk environment but no luck. I've changed the load balancer's target group health check settings from "200" to "200-301" but no luck as well.

The link to the instructional guide is pasted here: https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/php-hawordpress-tutorial.html

I'm unsure if it's my error in setting it up or an application error. Any help would be much appreciated. Thanks.