If you have an old pledge that you didn't complete already active in your journal, you need to abandon it before picking up a new pledge.

Hmmm well the only other thing I can think of is that they reset at a specific time each day, perhaps try again later?

I know that isn't particularly helpful, just trying to think of what could be blocking you from picking up the new pledge.

Then that's prob what is happening, hopefully it will be fixed tomorrow.