When saving a Custom Rule with a local user, the following error is returned:
SID for 'LOCAL\USER' was not found: Some or all identity references could not be translated.
The only local users that can be included by name in a Custom Rule are the generic users such as Local Administrator or Local System. To specify a specific, local user, enter the SID for that user directly in the field.
wmic useraccount where name='Specific Account' get domain,name,sid