Before openning up the laptop unplug the charger.
The CMOS battery is commonly called a “coin” battery because of its size and shape and are available almost anywhere.
It is to the right of the main battery in the repair guide below:
Sony Vaio Fit SVF15N26CXB Battery Replacement
You will need to unscrew the main battery and press the button you normally press to turn on the computer to discharge all power. Then take the CMOS battery out of its holder and leave out for 15 to 30 seconds.
Put it back in its holder, but do not put the main battery in just yet. Plug in the charger and see if any lights come on and the computer boots up. If there are no lights the charger may be defective.
If it boots up O.K. then shut down and unplug the charger. Screw the main battery back in, reassemble the case and try to turn it on.
If it works you are all done.
If not then the battery maybe dead or defective. If you plug in the charger is it charging the battery?
And oh don’t even think of replacing any soldered components unless you have a few years of experience - unless you want to make a brick. <g>
Let us know how it goes.