Hi BioShin IT,
As per your description, please don’t worry we can work together to narrow down and resolve the situation.
Based on my test result, check boxes that I can click to fill with an "x” after select Protect Form, when I click a check box, it is filled with an "x" in Word for mac OS.
The checkboxes in Word for Mac's interface are Legacy Form Fields and we cannot change a change the style of checkbox in Word for Mac. Regarding to this change the style of checkbox using form control, you may provide the feedback via Microsoft word app, to related team so that they can get notice about it. Your advice is very important to us. Refer to this article: How do I give feedback on Microsoft Office?
We can expect to be used in the future in Microsoft Word for mac.
Workaround:
If you just want to insert a checkbox with tick symbol static, as a workaround you may Choose the checkbox symbols for your customized bullets.
For reference: Make a checklist in Word.
Also, here is YouTube video for your reference: How to Insert a Tick (Checkbox) In Word - MAC
I appreciate your understanding and stay safe!!
Best Regards
Waqas Muhammad