Using the indirect function with combo boxes

You can't use INDIRECT with comboboxes. Whereas Data Validation List returns a string, which you can use further for INDIRECT. As you know, INDIRECT requires string for its argument. Combobox, instead, returns integer. You must link combo box with an any cell in the sheet, then you will have to use another formulas like INDEX, OFFSET, MATCH and many others to achieve you goal.