What is the legal ruling for someone who swore to fast on Mondays and Thursdays after committing a certain sin, then fasted on Thursday but did not fast on Monday due to being unable to have suhoor?

Abandoning sin is obligatory, and swearing an oath to abandon it further emphasizes this. This is called a "lajaj oath" (an oath made in anger or without full consideration). If the person who made this oath breaks it, they have the choice between fulfilling what they swore upon or offering expiation for an oath. If you commit the sin before the month ends, you can offer expiation for the oath by feeding ten poor persons, or clothing them, or freeing a slave, or, if you cannot find any of these, by fasting for three days. You can also fast this Monday, having already fasted last Thursday, to make up for the Monday whose fast was missed due to an excuse. So, beware of sins and strive to avoid them.