When upgrading to GP 2018 R2 you may have to add back the letter writing templates.
— Read on thedynamicsexplorer.com/2019/05/20/dynamics-gp-letter-writing-assistant-letter-templates-missing-following-upgrade-to-dynamics-gp-2018-r2/
Dynamics GP – Letter Writing Assistant – Letter Templates missing following upgrade to Dynamics GP 2018 R2 – The Dynamics Explorer
Install, Upgrade & AdminGP 2018
Archived post from 2019. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.