Cannot test SPFX webparts on my SharePoint tenancy

Your web part will not appear in the toolbox. Please make sure "gulp serve" is running in a web part project. Please refresh the page once "gulp serve" is running.

Run the below command to install the certificate, note that the certification error in the local workbench also disappears after this.

gulp trust-dev-cert

After this, you can run gulp serve to again see the local workbench and then launch the Office 365 tenant site to see if your locally developed webpart is available to be added on the workbench.aspx.