The requested URL can't be reached - MuleSoft Mock Service Error

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 

https://docs.mulesoft.com/api-manager/v/1.x/accessing-your-api-behind-a-firewall

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

https://docs.mulesoft.com/api-manager/v/1.x/cors-policy