This post fromMiniToolprovides some solutions for you.
This issue can be found on Windows 10/8/7.
The detailed information is as follows.

What causes the Microsoft Word isnt your default program issue?
Here are the feasible fixes.
Step 2: ClickOptions>General.
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Scroll down to find and click theStartup options.
Step 3: Uncheck theTell me if Microsoft Word isnt the default program for viewing and editing documentsoption.
Thus, you’re able to check the default apps.

Step 1: Press theWindows + Ikeys together to openSettings.
Step 2: Go toApps > Default apps> Set defaults by app.
Step 3: In the list, find your Word app and click it to chooseManage.

Step 4: Then youll see a list of file types Word can open.
Select Word for all file types its not the default app.
Step 1: TypeControl Panelin theSearchbox to open it.
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Step 2: Then, clickPrograms and Features.
Step 3: Find and choose Microsoft Office.
Then, click theChangeicon.
Step 4: Select theQuick RepairorOnline Repairbutton.
Step 1: Right-hit the file that you want to open and chooseOpen with….
Step 2: SelectChoose another app>More apps>Notepad.
Step 3: Check theAlways use this appbox and click theOKbutton.
Step 4: Then right-snag the same file again to selectOpen with>Choose another app>Word.
Step 5: Select theAlways use this appoption again, and click theOKbutton.
How to turn off dark mode on Word for Windows and Mac?
This post provides a step-by-step guide for you.
Continue to read this post.
it’s possible for you to try them one by one.