unable to delete KMS Customer managed keys.

Based on error message it is KMS key policy thing. In order to dig dipper you can indeed go through : https://docs.aws.amazon.com/kms/latest/developerguide/key-policies.html.

You can follow below steps to troubleshoot:

1. Check KMS key policy, check if role/user using which you are logged in having permission to manage KMS keys. Policy should look like below:

{
  "Sid": "Allow access for Key Administrators",
  "Effect": "Allow",
  "Principal": {"AWS":"arn:aws:iam::111122223333:role/ExampleAdminRole"},
  "Action": [
    "kms:Create*",
    "kms:Describe*",
    "kms:Enable*",
    "kms:List*",
    "kms:Put*",
    "kms:Update*",
    "kms:Revoke*",
    "kms:Disable*",
    "kms:Get*",
    "kms:Delete*",
    "kms:TagResource",
    "kms:UntagResource",
    "kms:ScheduleKeyDeletion",
    "kms:CancelKeyDeletion"
  ],
  "Resource": "*"
}

2. Also try to login using root account and check if you are able delete it.
3. If no user is not having permission to delete this key, best to contact AWS Support, they will help you.

Best Regards, Vikas