Problem
The membership sales task does not show up because it is inactive in the Pricebook.
Solution
By following these steps, you can see the membership sales task when creating a membership sale invoice.
Confirm sales membership task is active
Go to the navigation bar and click Pricebook.
In the side panel, click Services.
Use the search field to find the membership sales task.
If you can't find it, click Filters to open the Filters screen.
Filter by the Inactive status and click Apply.

Activate the membership sales task.
Click Actions and select the Activate option.
Or,Click Actions and select the View/Edit option.
On the Edit screen, enable the Inactive toggle to activate the item.
For more, see Activate a pricebook item.