Attachments cannot be added to a feature that has no existing attachments in the Map Viewer Classic attribute table when the Add option is disabled in the hosted feature layer editing settings.

Workaround

Workaround 1:

1. Enable the Add option in the hosted feature layer editing settings.
2. Open the layer in Map Viewer Classic with full editing control.
3. Open the pop-up of the feature record without attachments.
4. Click Edit in the pop-up and add attachments from there.

Workaround 2:

Use Map Viewer to add attachments.

Workaround 3:

Add attachments from the Data tab.

Steps to Reproduce