To edit Office documents with Microsoft Office you need to allow Microsoft Apps to access all files. Therefore go to the Android Settings and navigate to the Apps section. Select the Microsoft Office app with which you want to work (e.g. Word, Excel, PowerPoint or Office). Within the ‘App-Info’ screen select ‘Permissions’, then ‘Files and media’ and finally ‘Allow management of all files’. Microsoft Office Apps require unencrypted files to be stored by LAN Crypt in shared storage for editing.

Link: Set device storage permissions in Office apps for Android