The same problem occurred when moving external assemblies to the specflow.json file from app.config
.1) It turned out that I got a link to the assembly, which does not even have the nuances of specflow.
2) I renamed one assembly, but for some reason the old name was not replaced in the AssemblyInfo.cs file. I changed it manually, cleaned the project. Cleared the workflow cache (see previous answers on how to do this) and it worked.
So, check the correctness of all links and assembly names.
Greetings
Postscript Do not forget to set the option "Copy to output directory" for specflow.json: D
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