I can not update the latest version OS X on my macbook pro. Why?

What kind of MacBook Pro do you have? In order for your MacBook Pro to run OS X El Capitan, it must be a Mid/2007 MacBook Pro or newer. If you're not sure what model you have, read the instructions in the link I have included below to find out.

How to identify MacBook Pro models - Apple Support

I hope you find this information useful.

Best regards,

Austin