Hi Saba,
Entities that can display lists should be standard type, Task entity is active type, so it's timeline can't display active lists.
If you want the timeline control in one entity form to display the activity list, the entity needs to be associated with the activity entity:
--'Creating a new activity' need be enabled on the entity.
--Then current entity will be related to all Activity tables through 'Regarding' column.
--So activity types will be shown on the timeline of current table.
However, Task and Email are both Activity table!
--This feature is grey-out and can't be enabled, which is by-design.
As one workaround:
--Creating 1:N relationship between Task and Email.
Add one sub-grid related to email on the task form.
Then you can create emails in this sub-grid of task form..
I hope you can mark my answer verified if it is helpful! If you have any questions, please feel free to contact me.
Regards,
Leah