My setup is Sitecore 10.0 in CMS only mode. My installation also has the Sitecore Forms Extension for SC9.3-SC10.0 included.
I was adding forms using the Forms editor and got the point of adding Submit actions. Send Email is on the list but on selecting that action I was presented with a dialog box filled with the following error messages:
The partial view '/sitecore/shell/client/Applications/FormsBuilder/Layouts/Actions/CustomTokensForm/CustomTokensForm.cshtml' was not found or no view engine supports the searched locations. The following locations were searched:
/sitecore/shell/client/Applications/FormsBuilder/Layouts/Actions/CustomTokensForm/CustomTokensForm.cshtml
Description: An unhandled exception occurred.
Exception Details: System.InvalidOperationException: .........
This is a very clean installation in the process of an upgrade so I am puzzled. Checking the file system shows indeed that the named view file does not exist. I checked another 10.2 installation also on my system and that also does not include this file.
Any ideas on the cause of this failure? Answers below