That is L for Learner so in theory the car is being driven by someone who has not passed their test.

Can a learner drive this car alone without the trainer sitting beside him or her?

In Bangladesh, anyone can drive a car! No need for any license!!!

In Oman, it's a bit strict. There are specific cars for training purposes.

screenshot from Google search

A learner can only drive this car during his/her training and the instructor must sit in the passenger seat (right side) with a break in his/her control. Hence, Every instructor must have this car. When he drives this car for other purposes, the normal number plate is enough. When the learner is in the driving seat, additional plates will be put both in the front and back of the car, similar to one L in your picture.

Without passing the test, the learner can't drive other cars!