Fan stuck on low

It sounds like you either have a bad blower motor or the control circuit is faulty. To figure out where to look first you will need a multi-meter (even the cheap ones will work). https://www.2carpros.com/articles/how-to-use-a-voltmeter
Set it to the twenty volt or so scale. Now connect the leads of the meter to the two leads on the blower motor. The red on the meter goes to the purple wire on the motor.
Now turn on the key and watch the meter as you select the different blower speeds. On low you should see around six to seven volts, with the voltage going up every time you select a higher speed. On high you should see full battery voltage of around 12.5 volts.
If the voltage increases every time then the control side is working and the issue is in the blower motor. The common problem is a bad ground to the blower. To test the motor is simple. Run a wire from the battery negative terminal to the terminal the black wire goes to and another from the positive terminal and touch it to the terminal that the purple wire connects to. If the fan runs on full speed it is good.

If the blower tests good but the control side does not there are four main components to test. There are the low and high blower relays in the relay box under the hood, and the blower control switch and the blower resistor.