I'm using MacBook Pro with Sierra Version 10.12.6 and I am trying to upgrade to the latest version I could not do so. Can someone help me please. This has been given me lots of headache

This is your best and actually only option:

How to upgrade to macOS High Sierra - "If you have hardware or software that isn't compatible with macOS Mojave, you might be able to upgrade to macOS High Sierra"

- https://support.apple.com/HT208969

You may wish to consider getting more RAM. The system requirements may not ask for more but your computer will be very slow once you try running more than one or two applications with only 4GB RAM.

It would really, really help if you provided information as to your exact model computer, including year (and things such as early, mid, or late). You may have several upgrade options, limitation and hardware modifications you may need to do in order to install different versions. Somebody may just paste a whole bunch of system upgrade options here but I prefer the more directed approach to solving this kind of question.

To get more information about your computer, go to the Apple in the upper left corner of any window, then "About This Mac", then "More Info..." Copy and paste the information here, but omit the serial number and Hardware UUID (if present).

Normally the stock reply is, "Take it to an Apple Authorized Service Provider". That said, with a computer that age you can save a lot of money doing it yourself unless you really feel terrorized by using a screwdriver. ;-) It is possible an AASP wouldn't even be prepared to work on something that old.

I'd visit the macsales.com = OWC web site and use their upgrade information to see what is possible and for instruction videos. I'd make sure you buy your RAM from them or Crucial as sometimes similar specification sold for PCs doesn't behave well in Macs which are picky about quality.

It is also possible your computer still has a spinning hard drive instead of a solid state drive (SSD). A SSD will make it a lot faster and while you're on the OWC site you can check upgrade prices. A speed boost is nice but if you're thinking you may be computer shopping in a year or two you can decide if it is worth it now. It's not brain surgery replacing it but a bit of a hassle transferring from drive to drive, in particular if you do not have a backup (which should be your priority #1 before upgrading the system software).

What are the specs on your MBP, especially the date? Must be Mid-2012 or newer. (Apple menu > About this Mac)

macOS Catalina - Technical Specifications

Yup, the latest you can upgrade to is High Sierra.

Actually for RAM you might not even need a screwdriver. I don't think I needed one to do the RAM on my '08 MacBook.

Glad to help. :-)