ASP.Net- C# sample code to FAX PDF documents

I am new to FAX sending using RingCentral. I got the above error message while working with the code.

I added the below line to web config.

<add assembly="'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51" tagPrefix="extension"/>

still, the error exists.

Do we need to replace the clientid and clientsecret id with our registered username and password?

Please help me to proceed with it.