I was getting this error in visual studio 2019 each time I tried to add the identity scaffolding to a Angular Web application which was causing me much annoyance.
In the end I noticed that there was a mismatch in the installed project assemblies – most were 5.0.3 but Microsoft.VisualStudio.Web.CodeGeneration.Design along with a few others were 5.0.2.
At this time, there was no nuget 5.0.3 so, I changed all the project assemblies to match and ‘hey presto’ I can add the identity scaffolding to my angular app!
Only problem is – I really wanted to use Vue;)!
Anyway, I hope this helps someone else out as it had previously led me back to using PoMVC!