Forge version: 14.23.5.2814
Twilight Forest version: 3.9.851
Link to crash log: (please use a paste site such as gist, do not attach the .txt or paste the log inline)

Steps to reproduce: defeat the Phantom Knights
1.

What I expected to happen: I expected to complete the advancement to move onto the Ur-Ghast, Clearly I have the trophy on my hotbar to prove it

What happened instead: No advancement

Known issue, already fixed in dev (89fae3c).

ok, but how do i fix it in my world?

There hasnt been a new version of Twilight Forest since Jan 12th 2019, so this will be fixed in next version?

Yes, this will be fixed in the next alpha version released.