Is there a way to run a load test by throughput instead of setting the maximum number of concurrent sessions?
Sure, click the arrow next to Max Concurrent Sessions, select Max Requests Per Second and set the number you wish.
The message you've got means that you've reached your plan's maximum credit. Please go to Settings -> Billing -> Change Plan and upgrade. If you don't have access to the Billing tab, contact your team admin who will be able to buy more credit.