Why can’t I install on my MacBook Pro (retina,13 inch,early 2015) 10.11.6 macintosh hd

macOS 10.11 is no longer supported by Apple or by most third party developers in the App Store. Most likely the apps in the App Store will require a much more recent version of macOS (most likely macOS 10.15+). You should still be able to redownload & install any older versions of apps you previously purchased in the App Store.

Lucky for you, your laptop is able to run up to macOS 12.x Monterey, but you may first need to upgrade to macOS 10.13 before being able to upgrade to macOS 12.x. Even if you can upgrade directly to macOS 12.x, it would be advisable to first upgrade to 10.13 so that you might have a chance of also upgrading your third party apps to 64 bit versions as well since macOS 10.15+ no longer supports 32 bit apps. Ideally you should upgrade the apps to 64bit version prior to installing macOS 10.15+. Otherwise it is recommended to uninstall your third party 32bit apps before upgrading since you won't be able to easily do so after macOS 10.15 is installed.

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

Here is an Apple article with instructions for getting various versions of macOS installers:

How to download and install macOS - Apple Support

I would recommend to create a bootable macOS USB installer using the instructions in this Apple article just in case something goes wrong so you have more options:

Create a bootable installer for macOS - Apple Support

Definitely make sure to have a good backup of your data before making any attempts to upgrade macOS. You should always have frequent and regular backups all the time anyway.

haram66 wrote:

i Can’t install things idk why I also have an account on AppStore but I don’t seem to know what problem it is?

What "things" can you not install? What happens when you try to install "things"?