This issue is based on performing authentication with Magento. Using the Bearer Security Scheme, the API returns a String value, although the API specifies application/json. This results in a failure to get the authentication token. The following Curl command will show provide clarification of the issue.
curl -X POST "https://demo1-m2.mage.direct/rest/V1/integration/admin/token" \
   -H "Content-Type:application/json" \
   -d '{"username":"demo", "password":"demo123"}'
The result of this command is (for example):
"77gdagby0gs1f3xff03u4q8wt9gf3n55"% 

You can reproduce this by adding the attached magentoCartService.yaml to a light-module's restClients folder, and test the function in the Rest Client tool.