Error '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

Because the Related To field is reserved for post-conversion use, the fix is to create a custom Quick Action for tasks on the Lead page layout that excludes the Related To field

This behavior is expected and working as designed as the Related To field needs to be available for population once the lead is converted.
Salesforce reserves the Related To field on lead tasks for after the lead converts into a Contact, Account, and optionally an Opportunity.
By default when using Lightning Experience the default global action for creating a new task is used for the lead object. 

To avoid this error in Lightning Experience, create a custom quick action to create a task and remove the Related To field from the layout or make the Related To field read-only on the layout. 

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.