Everything is ready except for one last crucial factor; literally "all is ready except for the east wind," referring to a story from the 三国演义 sān​guó​yǎn​yì where a fire attack required a specific wind to succeed.

everything is ready except for the one final key condition; literal sense refers to the Battle of Red Cliffs where all preparations were made except for the favorable east wind

everything is prepared and the only thing missing is the most important factor; a metaphor for having all resources ready but being unable to proceed due to a single missing necessity

everything is ready except for one final key element; all is prepared except for the one most important thing