Execution Error: Execution error : System.Xaml.XamlObjectWriterException: Cannot create unknown type '{http://schemas.uipath.com/workflow/activities}SendMail'

[Dominic]

Cannot create unknown type error comes when the activity that is mentioned after the schema URL (https://schemas.uipath…) i.e SendMail (that is part of UiPath.Mail.Activities) is not available

From the error message, seems that the robot machine doesn’t have the exact version of the UiPath.Mail.Activities which is added as a dependency for that project.

Steps to resolve:

1. Open project.json of the project & verify the UiPath.Mail.Activities version
2. Please check if the robot machine has the same version of that package in %userprofile%.nuget\packages (if the robot version is >= 2018.2) or %localappdata%\uipath\activties (if the robot version is < 2018.2) or your customized package installed path location (if the robot version is >= 2018.4.4)
3. If the above directory doesn’t contain the version, to resolve temporarily copy the exact version of the UiPath.Mail.Activities.{versionNumber}.nupkg from a working environment\machine and place it in <Installation directory of UiPath Robot>\packages folder
4. Long term solution is to whitelist the online feed (myget.org & nuget.org, *.vo.msecnd.net) where the packages are automatically downloaded from the below feed as shown:
   

Detailed explanation: Cannot create unknown type - V2018.2.3 Onwards - Troubleshooting steps