Access to AWS Cost Explorer is denied

You should check whether the access configuration is correctly set up. Have a look at here: https://docs.aws.amazon.com/cost-management/latest/userguide/ce-access.html

Once this has been set up, you need to have a role with permissions to the cost explorer (in case of issues, you can have a look at: https://docs.aws.amazon.com/cost-management/latest/userguide/security_iam_troubleshoot.html#security_iam_troubleshoot-no-permissions)

Hello.

Have you enabled billing information access for IAM users using the steps in the document below?
If you do not enable access to billing information, you will not be able to access it even if an IAM policy is attached.
The steps in the following documentation must be performed as the root user.
https://docs.aws.amazon.com/IAM/latest/UserGuide/tutorial_billing.html#tutorial-billing-activate