Facebook appears to let me only use custom audiences but not lookalikes. Anyone know if you can or if this is something related to my account?
Not just your account, I can't use audience insights on lookalike audiences either
It is not possible. Lookalike audiences are not built from explicit user actions such as from a mailing list, FB pixel events, etc. so are probably excluded for that reason -- this may change in the future, which would be great, but don't hold your breath.