MacBook Pro Shuts off or won't start up while enough battery

That Mac's battery is malfunctioning. Review If you see Service Recommended on your MacBook Air or MacBook Pro - Apple Support, particularly those under "If your battery isn't charging." Follow those steps.

The context of your question implies you may have purchased that Mac "used". If that assumption is correct, its previous owner may have chosen to replace its battery with a poor quality aftermarket one. In any event it sounds like it needs to be replaced.

If it is in fact original, it's quite old and most likely in need of replacement anyway. We can reasonably expect to get about seven useful years from a new battery, at most.