i have just done that a league member should do.语法错误

What a league member should do 在这里是done 的宾语,这是个宾语从句.而在这个从句中what 做 do 的宾语；如果用that 引导 就应该是定语从句,that 应该在一个名词之后.你可以分析理解下这两句：
1.i have just done what a league member should do.
2. i have just done something that a league membet should do.

本句中a league member should do是做have just done的宾语从句。that引导宾语从句时，句子结构完整，也就是主谓齐全，并且该有的宾语也齐备。本句中should do缺少宾语，所以不能用that引导。what 引导宾语从句时，what本身可以在从句内部充当主语或者宾语和表语，同时在主句中也充当一定人成分。本句可以分解为先行词+定语从句的结构，理解起来就更方便了。