Translation:
The book of Sibawayh is a book that normal people cannot do (OR read OR write).
Meaning of يقدر upon the context is يقرأ (read).
The reason for that choice:
Since a book is a thing has two functions one that it is written and the other function is that it can be read by readers and because people are mostly readers not authors.
So, يقدر here may means the reading.