You must be:
Logged in as an admin of the Jira Server instance.
Go to :cog_icon: > Add-ons.
Click Find new apps.
Search for Hierarchical Calculation Field.
Click Try free to begin a 30-day trial or Buy now to purchase a license for "Hierarchical Calculation Field".
Enter your information and click Generate license when redirected to MyAtlassian.
Click Apply license.
Once the purchase process is completed, the plugin will install automatically.
The Add-on "Hierarchical Calculation Field" will be displayed under User-installed add-ons.