operating system upgrade

It's telling you to connect the charger even though the charger is already attached?

If so, try a different port, a different wall outlet, flip the charge connector over and reinsert. You may not actually be "making" an electrical connection.

I would try a different port. I would also make sure that you have enough wattage for your MacBook. If both of those things don't work, try restarting the Mac and installing the upgrade again.

If that doesn't work, something may be wrong with your MacBook (either charging port, battery, or a chip).

As @ ku4hx suggested and make sure the Original Apple Charger and Original Apple Cables are being used.