I’m struggling to understand what this rule is doing. Is it identifying when license/license is being used as a noun?license must be spelled with a "c" when used as a noun in British English. Use licence. AmE/BrE: license/licence (noun singular) Please show me your license.
You see the message only if you select the text language British English. In British English, the noun is ‘licence’ not ‘license’.
The message comes from the rule LICENCE_LICENSE_NOUN_SINGULAR. You can see the XML code for the rule on https://community.languagetool.org/rule/show/LICENCE_LICENSE_NOUN_SINGULAR?lang=en-GB.