Is the sentence "I will see the doctor at 2.00 this afternoon so I can't go to the lecture" right?
I am confused about the difference between
"I will see the doctor at 2.00 this afternoon so I can't go to the lecture"
and
"I'm seeing the doctor at 2.00 this afternoon so I can't go to the lecture."

The grammar book says "i'm seeing" is right
but I can't understand why "I will see" is wrong~orz

Question

Is the sentence "I will see the doctor at 2.00 this afternoon so I can't go to the lecture" right?
I am confused about the difference between 
"I will see the doctor at 2.00 this afternoon so I can't go to the lecture" 
and 
"I'm seeing the doctor at 2.00 this afternoon so I can't go to the lecture."

The grammar book says "i'm seeing"  is right 
but I can't understand why "I will see" is wrong~orz

Answer

They are both correct. Sometimes present cont. is used instead of future, and it is more common,  but the future tense is also correct.

لا تفوّت فرصة تعلّم لغة جديدة وأنت مرتاح في منزلك. تصفّح مجموعتنا المختارة من مدرّسي اللغات ذوي الخبرة وسجّل في درسك الأول الآن!