See cost and usage as a User

This is a common issue with AWS billing access. Even though your IAM user has AdministratorAccess and custom policies for Cost Explorer, there's a specific account-level setting that needs to be enabled first.

By default, IAM users cannot access billing information (including Cost Explorer) even if they have administrative privileges. This is a security feature in AWS.

To resolve this issue, you need to explicitly activate IAM access to the Billing and Cost Management console. This can only be done by the AWS account root user by following these steps:

1. Sign in to the AWS Management Console with your root user credentials
2. On the navigation bar, select your account name, and then select Account
3. Scroll down to find the section "IAM User and Role Access to Billing Information"
4. Select Edit
5. Check the "Activate IAM Access" box
6. Choose Update

After completing these steps, the system will display a message that "IAM user/role access to billing information is activated." Once this setting is enabled, your IAM user with the appropriate permissions will be able to access Cost Explorer and other billing information.

The error message "IAM user access not activated" specifically indicates that this account-level setting hasn't been enabled yet, rather than an issue with the IAM user's permissions.
Sources
User cannot view the billing dashboard | AWS re:Post
Access denied even though ive set permissions | AWS re:Post
Billing: Access Denied | AWS re:Post
Cannot access cost explorer despite being admin | AWS re:Post

This worked perfectly, thanks for the quick response.