I am getting the message “order total $x.xx + open position total $x.xx must not exceed maximum position size $x.xx” when trying to make a trade. What does this mean?

This means that your order is violating the position limit rule set by the professor. Position limit is the maximum % of your portfolio value that can be comprised of 1 single security. So for example if your position limit is 25%, then no single security can make up more than 25% of your portfolio value. Professors can also choose to set different position limits for different types of securities when creating their class.