eXtyles, like many other third-party Microsoft Office add-ins, was written as a 32-bit application, because it predates the 64-bit version of Office (introduced in 2010). 32-bit applications are not compatible with 64-bit versions of Office. You can find technical details about this limitation in these Microsoft articles: Compatibility Between the 32-bit and 64-bit Versions of Office 2010 and 64-bit Editions of Office.
For most Office users, and especially for those who primarily use Word and Outlook, 32-bit and 64-bit Office are functionally identical. However, the 64-bit architecture does provide additional power that can benefit power users of Excel and/or Access. IT departments may also prefer to install 64-bit Office because of its default security features.
As 64-bit Office has become more widely used, our developers have been working on creating a 64-bit-compatible version of eXtyles. Our goal is to have this available to customers in 2021. We thank you for your patience while this major update is in the works!
Not sure what version of Word you're using? Learn more here.