Variables cannot be read from the OPC server (CODESYS)

I'm working with Codesys V3.5 and the latest Version of Matlab (R2019b). After installing the OPC Toolbox in Matlab and the OPC Server from CODESYS, it's not possible to read the variables with the "OPC Read"-block in Simullink. The client is connected to the server. I don't use any graphic elements, like it's mentioned in a post before.

Is there any solution to this problem ?

[Kavya Vuriti]

Is there an OPC Configuration block in your model? You must have an OPC Configuration block in your model to use the OPC Read block. If it is present, what is the exact error or warning displayed?