I'm pretty sure you're not using JDK 8 to build the project. I just tried with JDK 15 and got the same error. If you use Terminal, set [font=courier new]JAVA_HOME[/font] environment variable to ensure Ant uses the right JDK.

Do I have to put the sweethome3d.jar in any specific folder to aid the building?

No, just leave it in its default place, but SweetHome3D and SweetHom3DJS projects must be in the same folder.