I have been committed to an AWS Foundational Certification recently after procrastinating for a long time and upgraded my level of preparation to eye the Developer Certification.
I found that my non-root administrator user doesn't have access to Billing dashboard, even after adding BillingFullAccess Policy explicitly, as mentioned in the most returned post while googling the issue. I even tried to delete and create a new user with permission as mentioned in above link.
But, here is what you need to do.
You need to activate IAM Access from your root account settings.
Login to root account and go to My Account from top navbar.
Scroll Down to 'IAM User and Role Access to Billing Information' and click on Edit.
Check the box and Update the 'Activate IAM Access' and you are done!