Q: I have a macbook pro with os x 10.6.8 ,but I cannot download OS X El Capitan in App Store,why?

Without more information on the exact build model year and specification of the MacBook Pro

you have, I can only surmise the unit may be too old to upgrade to El Capitan from App Store.

• How to identify MacBook Pro models - Apple Support

If that is the case, the other option (after identifying the computer) would be to see if it is of the

correct specification to be able to run either OS X Lion 10.7 or OS X Mountain Lion 10.8; both

of these are listed for sale in the Apple Store online (for about $20 USD) and the product would

be in the form of a redemption code that could be used in the App Store to procure a download.

• OS X Lion (10.7) - Apple Store:

http://www.apple.com/shop/product/D6106Z/A/os-x-lion

• OS X Mountain Lion (10.8) - Apple Store:

http://www.apple.com/shop/product/D6377Z/A/os-x-mountain-lion

And if your computer is one that can use either of these, or may use the higher number later OS X

you would not need to purchase 10.7 if the computer can run OS X 10.8. Some older model Macs

could use one or the other, but if your computer can use Mountain Lion 10.8(.5) get that one since

the system is newer and had better reviews from users than Lion 10.7(.5) -- also note that each of

these also have an update download to complete them to the last step, a Combo update, after the

main OS X is installed. This last step should appear in Software Update or the App Store after you

have installed either of those.

Then use these links to update to last step available for either, after appropriate OS X upgrade is installed:

• Lion Combo Update for 10.7 - once installed:

Download OS X Lion Update 10.7.5 (Client Combo)

• Mountain Lion Combo Update for 10.8 - once installed:

Download OS X Mountain Lion Update v10.8.5 (Combo)

If for some reason your computer has other issues that prevent it from using El Capitan, you may

have to see if an Apple Store can help determine the problem and help resolve it. If you are near

an official Apple Store you may contact them and ask about setting appointment with Genius. If

there are no official Apple Stores, an authorized Apple service provider or reseller with service

department may be able to help you. A fee may be assessed for the technician shop time.

• Make an appointment at an Apple Retail Store Genius Bar
• Find an Apple Authorized Service Provider

{Was there a warning message when the El Capitan did not download? ~ If so, what did it say?}

Without more information, there are several possibilities as to what may be going on, to guess.

You could look in the About this Mac in the Finder menu (under Apple icon) and see the info that

will appear to tell you about your computer with some detail via More Info button. This may help

to determine the computer model and build year, and to what system it may be upgraded.

Good luck & happy computing!

What exactly happens when you try?

If your MBP is a mid/late 2007 or newer, El Capitan is compatible.  If it is older, then El Capitan is not compatible.

Ciao.

Hello, again:

Here's a link to El Capitan w/ minimum requirements for OS X 10.11, I'd forgot

to bookmark at an earlier time; and so also missed including this in earlier reply:

• OS X El Capitan - Technical Specifications

There are hardware limits in certain older models in sections that are not able

to be upgraded to run the latest OS X, so that may affect your hope to run it.

Information such as http://everymac.com covers most Macs & their range of OS X

and hardware configurations. Also, http://mactracker.ca has a database download to

see these specifications in an app, that are helpful if one is to attempt to upgrade.

Good luck & happy computing!

Make certain that you have pertinent EFI firmware updates applied before attempting to install a later version of OS X that may make the expected firmware revision level a pre-requisite for installation.