I can't complete the challenge because the field name creates an error, Days_Since_Last_Update__c

Error: The custom field name you provided Days_Since_Last_Update__c on object Case can only contain alphanumeric characters, must begin with a letter, cannot end with an underscore or contain two consecutive underscore characters, and must be unique across all Case fields

I can't complete the challenge because the field name creates an error, Days_Since_Last_Update__c

Error: The custom field name you provided Days_Since_Last_Update__c on object Case can only contain alphanumeric characters, must begin with a letter, cannot end with an underscore or contain two consecutive underscore characters, and must be unique across all Case fields