Error encountered while accessing Mock Service
The requested URL can't be reached
The service might be temporarily down or it may have moved permanently to a new web address.
for details refer
For testing purpose as quick fix "Bypass the proxy". Select option next to "Mock Service" stating " API is behind a firewall"
Actual fix before you implement it in Test/Production environment is to implement and apply a CORS Policy for your browser to be able to load the resources in the API behind your firewall and the Anypoint Platform’s browser tools