Hi [MENTION=10108383]EmmaE99[/MENTION] That is weird. It looks like you are trying to use the mobile BA website. You should still see that option to select a companion voucher from the mobile website.

On the mobile website (not the app) navigate to the BAEC screen where it says Welcome to your Executive Club, EmmaE99 and you select the My Executive Club Menu where it lists your Avios and tier points if you scroll down the page you should see a section titled My eVouchers and it should list the voucher below that with a radio button Redeem.

If you can't see it there have you got access to a laptop or desktop and tried to see what you see when you access BAEC there?

Despite you claiming your voucher is still valid it sounds like it has expired and is no longer available for you to redeem.

The only other suggestion I can make is to phone BAEC - they should be able to see it and book flights for you.