Most often, this is due to signing up for Everplans using a different email from the one that your Deputy invitation was sent to.
Please confirm with the person who invited you that the email address they used for your invitation is the same one that you signed up with.
If this is the case, you should ask the person who invited you to add you as a new Deputy with the correct email address.
Once that occurs, you will receive an email prompting you to log in and accept the new Deputy invitation.