Can I update my 2015 MacBook Pro with macOS 10.11.6?

An early 2015 through 2016 MacBook Pro can be upgraded to the latest version of Monterey macOS 12 but no later. See: How to download and install macOS - Apple Support

Be aware that if you upgrade to Catalina 10.15 or later you will also need to update any non-Apple 32-bit apps. See: https://www.stclairsoft.com/Go64/

or

 https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html

FYI, please don't create multiple posts for the same issue. I just got done replying to your other post. It just causes confusion for everyone and wastes the time of contributors who don't realize someone has already answered your other post. I'm asking the moderators to delete the other thread.

Here is my reply from the other thread:

macOS 10.11.6 is very outdated. Just about all app developers have abandoned this OS many years ago.

If you want to run more recent versions of your apps (assuming there are newer versions available), then you will need to upgrade the version of macOS on this laptop to macOS 12.x Monterey which is the last version of macOS supported for this laptop. You may first need to upgrade to macOS 10.13 since this OS introduces new firmware necessary to utilize the new APFS file system used by all newer versions of macOS. Once you have macOS 10.13 installed, then you should be able to upgrade to macOS 12.x Monterey.

I highly recommend you first make a good backup just in case something goes wrong during the upgrade process. The upgrade process will involve converting the HFS+ file system currently being used by El Capitan to the new APFS file system.

I also highly recommend you create a bootable macOS 10.13 USB installer as well so you have more options available in case something goes wrong. Here is an Apple article with instructions for creating a bootable macOS 10.13 USB installer (there is a link within that article for downloading various macOS installers as well):

Create a bootable installer - Apple Support

Once you have macOS 10.13 installed, you will need to make sure all your third party apps are 64 bit versions. If not, then update the apps with 64bit versions now while you still can.....assuming you can find a 64bit version of the app which is compatible with High Sierra. Otherwise, I recommend uninstalling the 32 bit apps now while you still can do so. Follow the developer's instructions for uninstalling each 32 bit app. 32 bit apps are no longer supported beginning with macOS 10.15+. So if a third party app has no 64 bit version, then you may only want to install up to macOS 10.14 Mojave, but even this OS is very old and many current third party apps may no longer support it either.

32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support

Make sure to keep that bootable macOS 10.13 USB installer until you have completely upgraded the laptop to Mojave or Monterey. After that, I highly recommend you make a bootable USB installer for the version of macOS you settle on (Mojave, or Monterey) so if you ever need to reinstall macOS you will have it. Even though the macOS 10.13 upgrade will update the laptop's system firmware to support Internet Recovery Mode to access the online macOS installers......these online installers may no longer be working anymore, at least for older versions of macOS so having a bootable macOS USB installer handy is a good precaution.