If an Excel comment contains the text, "[Threaded comment] This threaded comment can be viewed, but not edited...," this is because someone else opened the file in a newer version of Excel, and they used the new threaded comments feature. Since your version of Excel doesn't have this feature, it can't show the threaded comment. As a result, Excel shows the comment unthreaded, in plain text format.
Threaded comments have a built-in Reply box that lets you and others have threaded conversations about the data. They are only available in newer versions of Excel. For example, Excel for Office 365 has the threaded comments feature.
Threaded comments in newer versions of Excel
Comments in newer versions of Excel, like Excel for Office 365, have a Reply box. When people reply, you can see several comments connected together, showing a virtual conversation in the workbook. Older versions of Excel do not understand this kind of comment, so it shows the thread in plain text format.
Comments in older versions of Excel
Comments in older versions of Excel don't have a Reply box. They are for adding annotations or reminders in cells.
Upgrade your version of Excel to one that supports threaded comments.
If that is not a convenient option, you can do one of the following:
Refrain from editing the comments. This is recommended because newer versions of Excel will remove those edits.
Tell other people to use notes, not threaded comments. Other people working on the file in Excel for Office 365 can use notes, instead of comments. For more information, see Threaded comments and notes. If they use notes instead of threaded comments, you'll be able to edit the notes in your older version of Excel.