Load Test - Constant Load Issue

I am using Visual Studio 2013 Ultimate for load test. My project is running on visual studio online. I set up 1 for constant load(user number) and select Based on the number of virtual users.

After I run the test, test is aborted and I' m taking the error message:"VS1550081: Test run execution aborted"

My test is running correctly for step load from 20 users to 50 users. Why do I take this error on constant load?