
Sorry, but there isn't necessarily a simple solution when you see that error message.
The message basically says that there is an XML coding error but it doesn't tell you if the error can be fixed relatively easily or if the document is broken beyond recovery. You will need someone who can take a look at the underlying XML.
I see you have posted the same question in the Microsoft Community at https://answers.microsoft.com/en-us/msoffice/forum/all/what-is-the-solution-to-the-error-message-the/61cc014b-a7c8-4632-bff0-a620413e9ce3 .