Is a combination of だけ + しか. These words have opposite meaning, but because しか must always benegative-only , it emphasis the meaning of 'really only this thing.' Transliterated it would be something like "does not 〜 anything outside of only 〜".
彼は野菜だけしか食べれません。 He only eats vegetables. lit. "He does not eat anything outside of only vegetables."
この試験は成功率が２０％だけしかない。 The probability of success of this experiment is only 20%.
lit. "The probability of success of this experiment is not anything outside of only 20%."
今お金はそれだけしかないよ。 Right now, this is all the money we have. lit. "Right now, there is not anything outside of only this amount of money"