You may receive the following error when trying to open a Microsoft Office file in the browser stored in SharePoint on-premise using Office Web Apps or Office Online Server, or in SharePoint Online using Office Online:
Sorry, you can't edit this document with other people.
This error can occur if another user has the file open in a Microsoft Office 2016 client application, such as Word, when they have one of the following registry keys set to 1:
For a user with Microsoft Office 2013, the following registry key may be set to 1:
When the value is set to 1, it disables the Cell Storage Service. When this is disabled, it in turn disables the co-authoring feature of Office. So when a user opens the file, instead of allowing multiple users to edit the file, it puts the lock on the file and prevents others from editing the file in the browser or client.
To resolve the issue, make sure the registry key value is set to 0 on user machines.