Is it permissible for me to travel alone, knowing that I am a 20-year-old girl and do not have a mahram, and I need to travel 120 kilometers to complete necessary financial transactions?

The basic principle is that it is impermissible for a woman to travel without a mahram, but necessity or a need that is equivalent to necessity is an exception to this. Therefore, if the questioner is compelled to travel, it is permissible for her to do so, provided she takes the necessary safety precautions, such as traveling at appropriate times and with trustworthy companions.