Did you create test manager account and link a test account to test manager account?
I suspect that you are using the customer ID of Ad account creating as part of creation of Google manager account. If you want to connect to this account(non-test) then the developer token needs to be approved.
In order to use unapproved developer token, we need to create test manager account and a test account under that. The client customer ID should be the test account's. Developer token is still from manager account.
I believe these are not test accounts, those are business manager account and an Ad account. As this developer token is not approved, you should be crating test account with your second google account as described in "Creating a test manager account" section of the article I have shared . So basically you will end up having a test manager account and a test account in addition to above accounts. The ID of this test account should be used in destination configuration.
These test accounts will be tagged as "Test Account" at Top right corner in ads.google.com. Please find the attached screenshot for reference.