Hi @이 정현(Amy)
Do you have the same issue when creating word document in your device?
According to your description, it seems you don't have edit permission for document, so they cannot edit the file and saved it normally.
In this case, you may try following tips to assign edit permission for your users so that they can edit the documents:
1.Go to the document library and go to Settings>Library Settings>in Permission and Management, click Permission for this document library.
2.In the permission page, open the SharePoint group which name is Site Members and permission level is Edit.
3.click New>Add users, Add users to this group>Invite people>enter Every except external users, then click Share.
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