Hi @yamamo-yu ,
I assume you have upgraded to InfraWorks 2020 from a previous 2018.x version.
The platform where to synchronize to has been changed to Bim360 docs starting with version 2019.
You need to get a license for Bim360 docs if you want to have the synchronized option activated in InfraWorks.
See also: http://knowledge.autodesk.com/article/InfraWorks-cloud-service-and-web-page-is-not-available
Regards,
Karsten.