Transferrable married couple’s allowance
Introduced from 6 April 2015 it would appear that HMRC’s on-line claim form is not currently accessible. Do not worry.
If you are within self-assessment it should be possible to make the claim on the return for the year ending 5 April 2016. If not you will still have until 5 April 2020 to make a claim for the year ending 5 April 2016.