I can't use some App Store apps I have subscribed to

Word is a Microsoft product. You need to chat with them if your purchase isn't allowing you what's promised.

Apple is not responsible for Microsoft's products working with macOS, Microsoft is. But this sounds like a problem in activation, again a Microsoft problem.

Hi, Word Powerpoint and Excel are apps by Microsoft, which are available on App Store but are not developed by Apple. You can contact Microsoft support for this issue. Moreover, if you still cannot activate Word, then you can check your receipt if you are paying with Apple Pay or if your card is linked to Apple. Then you can ask for returning the payment if you really can't activate Word.

Also, there are Applications that support Word formats on the App Store such as the: WPS, and other similar "word" applications.