AWS Lambda - 403 Forbidden Error when Attempting to Create Lambda

I am unable to use the AWS Lambda service and have not found any reasonable explanation as to why.

I can only attempt to create a lambda function from the Lambda > Dashboard using the "Create Function" button, since Lambda > Functions page only displays a banner with the text "UnknownError" (403 response to the GET request to "https://lambda.us-east-1.amazonaws.com/2015-03-31/functions").

Additional details are below.

1. Dashboard shows error box with empty message
2. API returns { "Message" : null } in response to the POST to "https://lambda.us-east-1.amazonaws.com/2015-03-31/functions" (includes "X-Amzn-Errortype: AccessDeniedException" as a response header)
3. The user has adequate permission to use Lambda (Full-Access)
4. Tried creating lambda using Node 18 runtime. Tried from template and from scratch with same result.