Joomla 4 User Manual
Please first make a selection from the list
This error message manifests on Administrator Forms for many non-core extensions even when an item has been selected.
This backwards compatibility error will be fixed in core for 2.5.1.
Install the plugin from this forum item 
To address this problem now so that your users do not have to wait for a core fix, update your Administrator forms and change name="adminForm" to id="adminForm". If backwards compatibility is required, simply use both values in the form definition. In future releases, Joomla will only support id="adminForm"
A patch has been created to fix the backwards compatibility in 2.5.1