A TWebClientDataset can not be put into Insert mode when a TWebDBTableControl is connected to it.
You should be able to reproduce this if you use the Demo Project in
\TMS WEB Core Demos\Basics\Dataset
Place a TWebDBTableControl on the form, connect it to the datasource and run it, then try to click on the + button in the dbnavigator: Doesn't work. Remove the TWebDBTableControl, Insert works again.
Using the latest version of WebCore and Delphi 10.1 Berlin.