If the first reason is the cause, please swap the owner of the record to an active user.
Reason 2: The "Modified By" field is missing from the module
If it is the second reason, please check that the 'Modified By' which module the field has been removed from. This field is needed for Burst SMS to run.
If the field is indeed missing, here's how to add it back in.
1. Click on
CRM settings.
2. Under 'Customization', click 'Modules and Fields'
3. From the list, pick the module that requires the 'Modified By' field. We'll use the Contact Module as an example here.
4. Pick a layout that will contain the 'Modified By' field. Burst SMS requires that at least one layout has the field.
5. In the layout editing field, find the 'Modified By' field under the 'Unused Items' section in the sidebar.
6. Drag the 'Modified By' field into the layout. Click save once this is done.
Once the 'Modified By' field is restored the 'Send SMS' widget should operate as usual.