Can I scratch if the extensions are itchy?

No!

The extensions can feel itchy on your scalp, either because you’re still getting used to the extensions or because there’s still soap in your hair. Itchiness is commonly experienced on and around the braid and at the top end of the extensions.
 

We strongly advise against scratching with your finger nails. Your scalp needs to get used to the weave and the pressure created at the ends of the braid.
 

If it becomes really bad, DON’T scratch. Instead, apply pressure with your fingers or apply a little moisturizer at the top ends of the extensions.