Unable to upgrade MacBook to macOS past OS X Yosemite

first, hardware upgrades will not change which macOS the machine is capable of running.

but, that MBP can be upgraded up to and including High Sierra. you can find the download and instructions here: How to download and install macOS - Apple Support.

if you are having issues upgrading, what happens when you try? are you getting any error messages?

The system requirements for OS X 10.11 El Capitan include 2GB RAM, so your 4GB should be fine.

What happens when you try to upgrade the OS?

FYI... update is only to a newer version of the currently installed OS, ie Yosemite 10.4 to Yosemite 10.5.

An upgrade will change to a newer, renamed OS; Yosemite to El Capitan.

The 2011 MacBook Pro can run up to macOS 10.13 High Sierra. I recommend you upgrade fully to that.

✅ Check the compatibility of your Mac hardware with newer versions of macOS software:

➡️ macOS 10.13 High Sierra is compatible with the Macs listed here (c.2009)

➡️ Download the macOS upgrade installer app:

Click > macOS 10.13 High Sierra.

Once downloaded, double-click the “Install macOS…” app now found in the Applications folder to begin the upgrade.

Please see > How to download macOS - Apple Support.