update mac os to sierra?

If you have a MacBook Pro from mid 2009 El Capitan is the last/ latest OS it can run.

Sierras minimum requirement for a MacBook Pro is that is mid 2010 or later,

How to upgrade to macOS Sierra - Apple Support

If your Mac can run macOS Sierra then it can upgrade to High Sierra which is still receiving security updates.

How to upgrade to macOS High Sierra - Apple Support

So tell me what happens when you try to install High Sierra, screenshots would be helpful.

I have OS X El Capitan 10.11.6 on my MacBook Pro. I am having trouble running certain apps on this system and am trying to upgrade to a more current OS. I tried to install Catalina and got a message that it cannot be installed on my system. So then I tried to install Sierra 10.13.6. I was able to download the app but it also won't allow me to install it on my system. How do I upgrade from El Capitan?

I get this message: "This version of macOS 10.13.6 cannot be installed on this computer." I have a MacBook Pro Mid 2009. I see that the Apple site's description of HighSierra says it is compatible with MacBook Pro from late 2009. Does this mean that the El Capitan OS is the latest compatible OS I can install on my computer?

Just checked the compatibility for MacOS Sierra and it's compatible with the MacBook Pro introduced in mid 2010 or later. So it is also not compatible with my MacBook Pro mid 2009. So my question still is whether El Capitan is the last/latest OS I can install on my MacBook Pro mid 2009. Thanks.

If your mac is good to install Sierra, you can also install HighSierra.

I have El Capitan on my Mac; not Sierra. I don't even think Sierra is available for installation. Only High Sierra seems to be available.

Sure Sierra is "available" make a serch in iTunes on your mac. It is not easy.

I saw your last post too late.

If you can see HighSierra for download, then you can find Sierra where I said. If not, ElCap is not bad either.