I am trying to leave BT to go to another provider. However, when they try to access my account to start the swap it says they can't find my account. This is two different providers and it worked yesterday. I'm eager to do it online as I use a cashback site and so will get some money back
That would be up to your new provider to sort out.
You can check yourself by logging into MyBT, you account should be visible there.
Check that you name and address details are totally correct, as any error there, would needed to be corrected first by BT.