Have a look in your Spam folder to make sure that the invitation email hasn't been identified as spam by mistake.
If you still can't find the email, please contact the person or place that invited you to make sure they entered the correct email address. People are human and sometimes a spelling mistake occurs.
Wait a few minutes, even if emails are digital. Sometimes it can take a few minutes before the mail arrives.
If you still can not find the invitation, contact the person or place you were invited by and ask them to "resend" your invitation.