1. A system construction assistance apparatus, comprising: a memory configured to store a first summary information and a second summary information; at least one display comprising a first display screen and a second display screen; and a processor configured to control the display to display, on the first display screen, a plurality of units, which are components connectable to slots of a programmable logic controller, each of the plurality of units belonging to one of a plurality of categories comprising a power source category, a central processing unit category, and a motion control category, such that the units are sorted by category, receive a first input to select one unit from the units displayed on the first display screen, based on the received first input, generate a system configuration diagram, and control the display to display the generated system configuration diagram on the second display screen, wherein, after the first input is received by the processor and before a next input is received by the processor, the processor is further configured to predict, on a per category basis which is a larger basis than a per unit basis, a category from among the plurality of categories to which a unit to be selected by the next input belongs based on a category from among the plurality of categories of the selected one unit and based on a plurality of first inputs received in a past, and control the display to display the predicted category as a candidate category, wherein the processor is further configured to: count for each of selection combinations comprising a first unit from a first category and a second unit from a second category that is selected before the first unit, a number of times the first unit and the second unit are selected in combination, control the memory to store the number of times in the first summary information as a count value for the respective selection combination from among the selection combinations, update the second summary information, which indicates per category selection combinations, based on the count value stored in the first summary information, and wherein the processor predicts the candidate category based on values stored in the second summary information.