This article will brief on how to make variables in variable editor as read-only in service portal.
A. Based on roles:
The variables can be made as read only based on the roles of the user as detailed in article KB0683863. However, this will make the variable read-only across the whole platform, CMS, service portal, etc.
B. In London release:
There are situations, in which user wants to make the variables in variable editor as read-only on service portal alone. In that situation, user can configure the service portal record's quick start config with below value as documented in the Service Catalog Variable Editors.
This quick start configuration applies only from London release onwards.
When you follow this approach, you might encounter the Known Error while submitting/Saving the form as detailed in the article KB0715633 - Service Portal buttons don't work on the first click.
C. Pre-London release:
If you are pre-London releases, you can also consider using the catalog UI policy as shown in below screenshot,
This screenshot shows that it is applicable only to requested item, you can also set it on catalog task / item based on your business requirement.
Any supported release.