Check Box Content Control/Check box form field/ActiveX check box are not supported in the Word Online.
If you open the sharepoint file in word online, this will not work, we suggest you open a file containing checkboxes in word app.
More information for your reference:
If this is not feasible for you, please feel free to reply.
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.