www.wysiwygwebbuilder.com

I'm trying to activate version 17 but I get a "The specific serial number is invalid".
However this is the version number I received for version 16.
What to do?

You cannot use the serial for WB16 to activate WB17 to do so you are required to purchase a "New" licence for WB17.
See the following url reading from
Are you upgrading from a previous version?
https://www.wysiwygwebbuilder.com/purchase.html

• I've purchased my license after June 21, 2021 where can I get my free upgrade serial number?

• I'm an existing customer, do I get a discount

Version 17 is an upgrade, so your serial number for version 16 will not work.

You can read more about the upgrade here:
https://www.wysiwygwebbuilder.com/howtoupgrade.html

I'm trying to activate version 17 but I get a "The specific serial number is invalid". However this is the version number I received for version 16.

Correct, because that is for version 16 not 17. You will need to pay to upgrade to version 17.