The most common one is because someone has opened the file with a version of Excel that doesn't support co-authoring. There are a handful of reasons why this error can occur. If you want to jump to where someone is working, select their picture or initials, and then select the Go to option. If you lose track of who’s who, rest your cursor over the selection, and the person’s name will be revealed. And on other people’s screens, their own selections will be green as well. However, your selection will always be green. If you see other people's selections in different colors, they'll show up as blue, purple and so on. If they're using another version, you won't see their selections, but their changes will appear as they're working. This happens if they're using Excel for Microsoft 365 subscribers, Excel for the web, Excel for Android, Excel Mobile, or Excel for iOS. You might see other people's selections in different colors. (You may also see their initials, or a 'G' which stands for guest.) You know you're co-authoring if you see pictures of people in the upper-right of the Excel window. When others eventually open the file, you'll be co-authoring together. With the file still open in Excel, make sure that AutoSave is on in the upper-left corner.

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