Manychat will not execute this function because by TOS and messaging policies you’ll be contacting a user outside of the alotted 24hr window in most cases. 

There are two workarounds:

The first:
Run ads specifically targeted to someone’s birthday.
Push this into a Messenger funnel.
Deliver your discount.

Second:
Create your MC funnel and collect the contacts’ names and emails to Google Sheets.
Use IFTTT or Zapier to then take those names and fill your email list (have a segmentation or list setup specifically for this import); or fulfill this step manually by yourself or VA
Set up a birthday flow in your email client that will send that coupon or discount the week of or on their birthday with an additional follow up reminder in case they hadnt taken action

This would be my personal approach. I use birthday funnels for my audiences for multiple reason. Hope this helps.

But where/when are you collecting their birthday? You only mentioned name and email on the Google Sheet. 

@Jilly B create a automation, in this you can ask your subscriber for their data( email, phone number, birthday date), and tell them you will send a birthday coupon.

If you need any help for this, contact me

Hi, @Jilly B!

You must create a custom user field To populate with the birthdates.

In your flow, you collect the date with a “Data Collection” step and select your custom field to receive the information.

It’s very similar to collecting names, phones and emails.

Then you send all the data to Google Sheets 

@Fabio Gaulke ok thanks! I’ll try this.