There is NO magic pill that wil solve and/or change/modify the 18 digits to 22 digits, as and if you do this:

cmd prompt >run as administrator>type wmic baseboard get product,manufacturer,version,serialnumber 

And you get an 18 digits S/N instead of 22 digits S/N (see caption below) then you can't do anything. The genuine Acer product for the A515-56 comes with an 11th Gen Intel cpu of either i3-i5 or an i7 and many motherboard models of this laptop, you could have either an unusual motherboard S/N with only 18-digit S/Ns or a nongenuine Acer product? The only people that can help you is Acer, who you have to contact, and you have to tell them all the details above and from which retailer that you bought this laptop from, and let them investigate and tell you if its a genuine Acer product? Good luck and hope this helps you out, as we are only an Acer community and have no ties to Acer factory and/or warranty.

Because the cmd serial number is the Acer mobo serial number and what your laptops number is, forget about the rest and go with the cmd serial number,