各位板上高手好
找朋友打遊戲時，朋友雖然想打，但打之前可能會「打預防針」說：我昨天沒睡好，今天
狀況可能不太好之類的，如何用英文去表達「打預防針」呢？
My description would be: A person who makes excuses ahead of time in case that
he fail afterwards. I would like to know if there’s a phrase or slang to bet
ter describe this. Thanks!

Like: just heads up that I might be in bad performance as had a bad sleep night?

(making a) disclaimer.「（做出）免責聲明」。用在口語上會帶點戲謔，但反正「打預防針」也沒有多正經，我覺得剛好。

making excuses

just so you know