How do i install os x on my mac.

Is the Mac OS X El Capitan installer, an old installer you downloaded a while a go.

At the end of last month (October) some of Apples online installers certificates expired

meaning they couldn't be used to install, often giving the error you have come across.

Apple have released updated copies of these installers, you can download the El Capitan

one here, How to upgrade to OS X El Capitan – Apple Support

Go to Section 4 and click on Download OS X El Capitan, the instructions are slightly different

than before so please make sure to read all the instructions in Section 4 to 7 carefully.

It could be that the age of your MacBook doesn't allow for updating beyond a certain level of OS. Click on the Apple logo in top left corner of screen.

in About this Mac, click on Software update. This may advise you as to whether your MacBook is updateable to the latest OS, currently Catalina

What is the latest update for MacBook Pro?

The latest version of macOS is macOS 10.14 Mojave, which Apple released on September 24, 2018. Apple releases a new major version roughly once every year. These upgrades are free and are available in the Mac App Store.