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! 
I believe the new check boxes, that are more like conditional formats rather than form controls, do work online. On the desktop they are set using
> Insert / Cell Controls / Checkbox
The checkbox appears as formatting within a cell that actually contains TRUE/FALSE. Applying further condition formatting can achieve other effects such as the red/green cells shown in the picture. Clicking on the cell still toggles the setting.
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