"You can't relate an account, opportunity, or other object to a task already related to a lead" when creating a Task on Leads in Lightning Experience
|Knowledge Article Number||000233142|
|Description||When creating a new task on the lead object in Lightning Experience, attempting to put a value in the Related To field results in the message:
You can't relate an account, opportunity, or other object to a task already related to a lead.
When creating a task on the lead object using the Classic Interface, the Related To field is grayed out and unavailable for editing.
|Resolution||This behavior is expected and working as designed as the Related To field needs to be available for population once the lead is converted. By default when using Lightning Experience the default global action for creating a new task is used for the lead object.
If you want to avoid this error in Lightning Experience you'd need to create a custom quick action to create a task and remove the Related To field from the layout or by making the Related To field read-only on the layout. For information on creating Global Quick Actions please visit: Create Global Quick Actions
Once your quick action has been created remove the default Create Task Global Action from the Lead Page Layout and replace it with your custom action.
By replacing the default global action with the custom action, users will no longer see or be able to edit the Related To field in Lightning Experience.