I was told to update my operating system on a MacBook Pro, Retina 13", 2015 computer, but find no update available

macOS Monterey is compatible with these computers If it appear from that list it Qualifies

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

Youu subject line included "MacBook Pro, Retina 13", 2015.

That model can be upgraded to Monterey. Please do as Phillips said and go to About this Mac and either verify you have that Mac. If not what Mac do you have?

Yes, up to and including Monterey per Mactracker ... early and mid.

What are you trying to do exactly when you get the message? Where are you "looking" to find updates?

Go to the App Store and search for mac OS Monterey