Welcome to the community.
Try all of the steps below, paying particular close attention to step 4:
1. Make sure you're accepting 'all cookies' if prompted when using ee.co.uk otherwise the voucher codes will fail.
2. Delete cache and cookies from your browser.
3. Try typing the voucher code, rather than using copy/paste.
4. If you've previously entered the voucher and clicked 'Back' on the browser, the next time you enter the voucher again it fails. If you enter a random word in the voucher field/box, delete it and then try adding the voucher code again it should work.
5. Make sure you have not selected a 'Good as New' deal, these are not eligible.
6. Have you selected a special deal, such as a 'Trade In' deal? try selecting another phone or plan to see if the code then works.
7. Vouchers only work on handset and SIMO, they will not work on other products such as tablets and watches etc.
8. Try using an alternative internet browser to access ee.co.uk like Firefox.
Let me know if it works.
Chris
