i can't open the codex

[我草让你找不到我]

it display a 'x' on the head when i press 'c'

[kid_zomb]

Hey, this may be because you haven't unlocked the Codex yet! You get it early on from a conversation with Achilles. If you previously could use it but now you can't, that would be more concerning -- if it's the case, please press F10 during gameplay to report it to us as a bug.

[Lord Nightcrest]

Also, remember that you can't open it before an encounter is finished.

[我草让你找不到我]

but i have send him several gifts, and i have talked with him many times.