That's the rub phrase
That is the biggest or most difficult problem (with the situation being discussed)
A: "Why don't you buy something to eat, man?" B: "That's the rub! I don't have any money left."
A: "You really should get your life together!" B: "That's the rub, man! I don't know what to do right now."
A problem or difficult circumstance
To have to accept some troubles or worries that must be handled by oneself
Completely controlling over a scenario or situation.
This is a line from Shakespeare's Hamlet:
"To die — to sleep.
To sleep — perchance to dream: ay, there’s the rub!
For in that sleep of death what dreams may come
When we have shuffled off this mortal coil,
Must give us pause."