Hello MarenDOT,
Welcome to the Microsoft Community.
I understand that your Excel checkbox functionality is not working and that checkboxes can be added but never properly checked, which is causing you inconvenience.
Until the issue is resolved, temporarily consider using a checkbox in a form control as a replacement.
Please refer to Form controls - Microsoft Support

Then for this checkbox functionality failure, the exact cause needs to be troubleshot step by step:
Try creating a new workbook and then doing the checkbox test to troubleshoot if it is related to permissions on a specific document, if this is happening to all workbooks it looks like it is an exception in the application. I suggest you try the following in turn:
1. Enable Safe Mode
Launching Microsoft Office apps for Windows in Office Safe Mode ensures that it's safe to use the app if you encounter certain issues. This feature is useful when you need to open a file that cannot be opened in normal mode due to an application crash, or when you need help identifying an external program or extension add-on that may be causing problems at startup.
This method works for most versions of Office on a Windows PC:
Right-click the Start button (lower-left corner) in Windows and click Run.
In the Run box, type the following command:
For Excel: Type excel /safe, and click OK.
See if checkbox works normally.
2. Repair Office
If Microsoft 365 or an Office application (such as Word or Excel) isn't working properly, you can try to fix it. Try the “Quick Fix” first, if that doesn't work try the older and more thorough “Online Fix”.You may need to restart your computer after you're done.
For more information, see: Repair an Office application - Microsoft Support
3. Reset Excel
This will reset Excel to its default settings and may fix some formatting issues.
First, close all running Office programs.
Press the Windows + R key combination to open the Run dialog box.
Type regedit and press Enter to open the Registry Editor.
In the Registry Editor, go to the following path:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel
In the right pane, find and select the item named Data, right-click, and select Delete.
Close the Registry editor.
Disclaimer: Modifying registry or workgroup subkeys is generally intended for power users, administrators, and IT professionals. It can help fix some issues, however, serious problems can occur if you modify the registry incorrectly. So, make sure to follow these steps carefully. For added protection, back up the registry before modifying it. You can then restore the registry if a problem occurs. For more information about how to back up and restore the registry, click How to back up and restore the registry in Windows - Microsoft Support to view the article.
4. Reinstall Office
If none of the above methods work, try reinstalling Office.
5. Create a local account
If the problem persists, please refer to this link to create a new local account to reconfirm the situation.
Please refer to this link to create a new local account to attempt a fresh install and login, this will generate a new user profile to resolve the issue if it occurs in the current user's environment.
Manage user accounts in Windows - Microsoft Support
I hope this is useful and please feel free to ask any questions or share any result.
Best regards
Ian - MSFT | Microsoft Community Support Specialist