I think you are getting this exception because, "Expression result" in your 1st image is "false". So it will not execute "If yes" block. instead it will execute "If no" block.
In Order to make it execute your "If yes" block and send email to respective user, you need to make the condition from your 1st image to
Please follow below links to make your condition correct:
Compare dates - Sharepoint Calendar.
Is there an expression in Flow like =TODAY().
Update(as you corrected formula):
Use below formula for your condtion to check if today is the BirthDay of user.
@equals(formatDateTime(items('Apply_to_each')?['BirthDay'], 'YYYY-MM-dd'), formatDateTime(utcNow(), 'YYYY-MM-dd'))