Inserting a checkbox into Excel Online

Hi, EveSC1300 , I was facing the same problem as you after I researched and found a solution for this problem, I hope it will help you, the check box is not checked box by ticking, but you can create it this way as a drop list: 

Step 1: Click on the Spreadsheet field you want to add the check box.

Step 2: From the ribbon menu click on Data --> Data validation.

Step 3: Click on Setting --> Allow --> List --> Source copy this "☐, ☑" and paste it inside the field --> click Ok.

Step 4: Align the text into the center, now you can see, that it is worked.

Good luck!