Scam

I actually referred my friend a new sim with my link, she also bought the sim through that link and started using it.
But I haven't got anything yet why....
At first the app showed me that i ll get £40

How long ago was the purchase Siri29? There is a waiting period before you get your Reward. More information and FAQs here.

It's been more than 10 days of purchase and my friend started using it from sep 23 or 24.

Siri29 I've pulled the below from the Refer and Earn FAQs page. You'll need to wait the full 60 days then follow the instructions in the email you'll receive. More information can be found here.

We know it’s hard to wait! Here’s the deal:

Your friend needs to activate their SIM and make two consecutive monthly payments within 60 days of activation. Once that's confirmed, you'll get an email letting you know your reward is ready. Now we’re in business.

The clock starts from the day they put the SIM in their phone and start using it — not from when they signed up. Deep breaths.