I want to install macOS Monterey on my MacBook Pro (Retina, 13-inch, Early 2015). But its not able to find the version on app store

It means your Mac could not find the installer.

• Have you tried the command to find what installers are available to you using:

Terminal Application and copy and paste the following command Without the Quotation Marks

" softwareupdate --list-full-installers "

The above will search for all available Full Installer of macOS and the output will list them.

Should a Error message be presented or No Full Versions are listed - this may confirm the Servers are still not work properly.

I entered the command and it returned:

Finding available software

Software Update found the following full installers:

* Title: macOS Monterey, Version: 12.0.1, Size: 12128428704K, Build: 21A559

* Title: macOS Big Sur, Version: 11.6.1, Size: 12428472512K, Build: 20G224

* Title: macOS Big Sur, Version: 11.6, Size: 12428553042K, Build: 20G165

* Title: macOS Big Sur, Version: 11.5.2, Size: 12440916552K, Build: 20G95

* Title: macOS Catalina, Version: 10.15.7, Size: 8248985973K, Build: 19H15

* Title: macOS Catalina, Version: 10.15.7, Size: 8248854894K, Build: 19H2

* Title: macOS Catalina, Version: 10.15.6, Size: 8248781171K, Build: 19G2021

* Title: macOS Mojave, Version: 10.14.6, Size: 6038419486K, Build: 18G103

* Title: macOS High Sierra, Version: 10.13.6, Size: 5221689433K, Build: 17G66

I entered

softwareupdate --fetch-full-installer --full-installer-version 12.0.1

and it is downloading now

Reports since Friday Dec 10, 2021, the Apple Servers are playing up and may still be, as we discuss now.

Depending on what version of macOS is presently installed - Catalina and above may work

Terminal Application and copy and paste the following command Without the Quotation Marks

" softwareupdate --list-full-installers "

The above will search for all available Full Installer of macOS and the output will list them.

Should a Error message be presented or No Full Versions are listed - this may confirm the Servers are still not work properly.

Terminal Application and copy and paste the following command Without the Quotation Marks

" softwareupdate --list-full-installers "

The above will search for all available Full Installer of macOS and the output will list them.

Should a Error message be presented or No Full Versions are listed - this may confirm the Servers are still not work properly.

The servers are running (as of last night, at least) and that will list available full versions for the computer.

The second part of that process is to actually download the version you want (there is only one for Monterey):

softwareupdate --fetch-full-installer --full-installer-version 12.0.1

If the below applies to this computer wherein the Original Internal Drive is an After Market drive >>

Solutions to pop out the drive and replace with the Original Apple Drive, install Monterey to that drive and then pop out the drive replacing with the After Market Drive and install Monterey to that drive. Credit for this method belongs to @ lllaass

There are reports appearing on the ASC Forums regarding failed installation of Monterey on older Qualifying Apple Computers. In most cases this is related to having a NON Apple Original internal drive ?  

The EFI Boot ROM included in Monterey is verifying for Original Apple Drive and if not found , may Balk or fail to upgrade to Monterey. 

Reports indicate the EFI BOOT ROM is Required to Flash the Computer and is a  requirement for a Successful Monterey Installation.

Is it available in System Preferences>Software Update?

where are you located?

Are you sure you have a MacBook Pro (Retina, 13-inch, Early 2015)?