There is no [EXPIRY_DATE] substitution. The substitutions available are those shown at the bottom next to the parameter Substitutions in all fields of invoice:.

Can you please give me some directions on how to add the expiry date to the footer of my invoices?

Officially you can't.

I have searched the forum and somewhere here you are saying that it is possible, but I cannot figure out what you mean:

I stated having the expiration date in the payment text which is displayed as the item being purchased.

You could use CB Queries incubator project which allows turning the results of a database query into a field then substitute that field at the bottom of your invoice with [cb_myfield] usages. You'd have to query the subscriptions database and pull the expiration date, but then there's no way to really know what plan to pull from the database (my best suggestion would be to get the latest by subscription date, but then older plans won't display the right duration!). It more or less can't be done easily by any means; we've plans to expand CBSubs substitutions further in later releases however.