Sorry to be the bearer of ill tidings, but there most likely is not much you can do. In a properly created form designed for multiple platforms the user should be able to complete editable areas without having to remove any protection. Protection not only prevents users from making unauthorized changes but also is a part of the mechanism that makes the form operate.
Without seeing the actual form it's impossible to be sure, but in all probability the form was created using a Windows version of Word which supports 3 types of fields:
- Legacy Form Fields which also are fully supported in Mac Word. If these had been used there should be no problem completing the form using most any version of Mac Word,
- Content Controls, which can be created only in a Windows version but will function in Office 2016 programs if the installation is fully updated. Confirm that your software is at version 15.31 or later, &
- ActiveX Controls, which are wholly unsupported on a Mac. If these have been used you may be able to complete the form using Word Online, but probably will have to use Word for Windows.
The only other option is to print the form, complete it by hand & mail it in. If it needs to be submitted electronically, scan the completed form as a PDF, then attach to an email or upload to the site.