各位板上高手好
找朋友打游戏时，朋友虽然想打，但打之前可能会“打预防针”说：我昨天没睡好，今天
状况可能不太好之类的，如何用英文去表达“打预防针”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）免责声明”。用在口语上会带点戏谑，但反正“打预防针”也没有多正经，我觉得刚好。

making excuses

just so you know