Beginners struggle with many skills especially if they are completely self-taught. I wouldn’t single out one skill as being more important than others that they lack. Every beginner is terrible at tennis in their own unique way!
Generally when teaching a beginner, a coach will focus on proper grips, use of offhand, moving to the ball properly, recovering to the right spot, stopping properly with correct spacing, coiling/uncoiling of the body with bent knees so that they are not just arming the ball, proper takeback and swing path, recovering to alert stance, reading the opponent, making proper decisions on picking trajectory, pace/spins, target spot etc. in some order of progression. Once some basic fundamentals of grip, footwork, spacing, body coil/uncoil, takeback/swing path have been established, coaches will focus a lot on spin generation as that is a key to then teaching how to increase power with control. This progression has to be done on FH, BH, return, serve, volley, overhead, BH slice etc.
So there is a lot that has to be learned even before you progress to how to play matches tactically, strategically, mental focus, routines between points, reading the opponent‘s game etc. Then you want to develop some shots and serves as weapons while you try to minimize all the weaknesses to a level that is not easily exploitable. You will run a lot of drills to improve consistency, accuracy, power, depth, footwork, hitting on the run, speciality shots like dropshots, topspin lobs, specific point patterns, specific shots with focus on serves, returns, +1 shots, putting away winners, coming to net, passing shots etc. So, it is typically a multi-year project to mold a beginner into a decent player who knows how to win a lot.
A common mistake that self-taught players make if they try to learn by watching online/pro videos is to focus too much on swing mechanics while not paying attention to footwork, body positioning/coil/uncoil, using the lower body including bending knees, stopping at proper spacing, stopping with the right pivot foot, transferring weight forward, extension etc. You cannot really execute a proper swing path if you get several of the other components wrong and that is why this kind of student can have a low ceiling.