What are my options if my Macbook Pro doesn't support higher than 10.12.6?

The macOS version you can upgrade to depends on the model and year of your Mac.

Click the Apple icon  at the top of your screen and choose About This Mac. 

These MacBook models are compatible with macOS 10.13 High Sierra:

• MacBook (Late 2009 or newer)

• MacBook Pro (Mid 2010 or newer)

These MacBook models are compatible with macOS 10.14 Mojave and 10.15 Catalina:

• MacBook (Early 2015 or newer)

• MacBook Air (Mid 2012 or newer)

• MacBook Pro (Mid 2012 or newer)

If your Mac does not meet the specs for a particular macOS version, your options are:

• Continue to use the Mac with the software you already have.

• Purchase a newer Mac that meets the specs of a newer macOS.

If $ is a concern regarding a newer Mac, you might look here at Apple Refurbished Macintosh computers.

EDIT - I realized now that your MacBook Pro is a 2015 model. That model IS supported by macOS versions 11, 12 and 13 - Big Sur, Monterey and Ventura. Baring some hardware problem, there should be no reason you cannot upgrade to one of these.

I am not seeing a "late 2015" MBP in Mactracker. but both the early and mid 2015 MBP can run upto and including Monterey. you can find the older macOS here: How to download and install macOS - Apple Support

but going from Sierra to Monterey, I would recommend at least upgrading from Sierra to High Sierra before attempting to get Monterey. and if you have the time, I would upgrade one macOS at a time all the way up to Monterey.

How to Upgrade  macOS:

STEP 1 - Ensure you have a current backup of your computer.

The macOS version you can upgrade to depends on the year your Mac was released.

Click the  (Apple) menu at the top of your screen and choose About This Mac.

Check the Compatibility of your Mac with newer versions of macOS:

➡️ macOS 10.13 High Sierra is compatible with the Macs listed here

➡️ macOS 10.14 Mojave is compatible with the Macs listed here

➡️ macOS 10.15 Catalina is compatible with the Macs listed here

➡️ macOS 11 Big Sur is compatible with the Macs listed here

➡️ macOS 12 Monterey is compatible with the Macs listed here

➡️ macOS 13 Ventura is compatible with the Macs listed here

❗️ – IMPORTANT – None of your 32-bit apps will work in macOS Catalina or newer. This includes MS Office apps and any other older software apps and drivers. These will require that you find 64-bit updates and/or replacement titles.

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

Use the free utility Go64 to check the 64-bit compatibility of your apps:

https://www.stclairsoft.com/Go64/

✅ - IF your Mac is compatible, then download the macOS upgrade installer app:

Click > High Sierra,  Mojave,  Catalina,  Big Sur,  Monterey  or  Ventura.

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

Please see > How to download macOS.

Before you upgrade, check the free space on your storage drive. Some older entry-level Macbook Pros shipped with tiny 128 or 256GB SSDs and may currently not have enough free space to allow the upgrade until some serious housekeeping is done.

I like my free space to be at least twice the size of the installer to allow for unstuffing the installer and any file swapping that goes on with a system upgrade.

Your late 2015 can run Monterey. Mine does so beautifully.

you're welcome. :)