It seems that your issue is on finding the "AdWords API Center" in the AdWords UI so you can generate a developer token. For the test manager account, you cannot see the "AdWords API Center" link, therefore, you can't generate the developer token from it. You need to create a non-test manager account (or production account) as you can see in the step 1 of this
guide.
Kindly follow the rest of the steps in the guide and let us know if you still encounter any issues. If any issue occurs, kindly provide screenshots that show the issue via Reply privately to author so I can further investigate.