All parallel tests are currently in use

Error Description
You have requested too many tests to be executed in parallel.

BrowserStack account allows a maximum number of tests/sessions to be executed simultaneously. It depends on the number of parallels purchased. You can use all the parallels you have purchased and rest sessions/test will be queued(equal to the number of your parallels).
To run parallel sessions greater than that number, wait for the previous tests to finish or upgrade your plan to add more sessions.