User is trying to attach a receipt to an expense entry in the Core Mobile app, but is unable to do so.
This can happen if you choose to Save and Submit when creating the expense entry and your security profile does not allow you to edit submitted expense entries. If you only choose to Save, you should be able to add an attachment to the expense entry and then submit it. You can also have the Core Owner or Admin user customize your security permissions from the Core web app and allow editing of submitted entries.
To do so, go to your main company menu > Settings > Security Permissions and click on the user whose security permissions you want to customize. On the Customize Profile dialog, check the Allow edit submitted entry option under Expense Entry and click Save. Check the Core Help for details on customizing security profiles.