Error using Rest Code

Hello everyone,
I'm new to heron. I'm following the Center for Spatial Research tutorial on github to install and test all components of Heron.
I already unblocked .dll files and the heron.gha file. testing the REST Geocode, I get this :
1. Solution exception:Méthode introuvable : 'System.String System.Net.WebUtility.UrlEncode(System.String)'.

What can I do about it ? What is it about ?
Thanks for all