An entry level MTB may not be tubeless-ready. You can look that up from the model name and number on the rim.
You can buy a patch kit for a few dollars and it will repair six flats. Carry one spare tube with you. When you get good at it, it'll take ten minutes to replace the rear, five minutes for the front. Five minutes longer for the second flat when you have to patch the tube.
If you're seriously adverse or unable to repair a flat in the field, consider something like Slime, but sometimes that causes as many problems as it solves. It can be a real mess if it doesn't seal a larger hole, or if you use it on high pressure road tires.