Verify your Advanced Rule (no parenthesis allowed and no GET or Email Invite Merge Codes, rule checker will fail even though these are VALID parts of an advanced rule)
A message will appear below while the rule is invalid and will clear itself when it is correctly formatted.
Example: 

'[%%21:Hello %%]' == 'Hello Nurse'
[%%2:%%] >= 2 OR '[%%264: Hello World%%]' == 'world' OR [%%5:%%] != 72