When tried to generate source code, there are error message below :

Message type: Error Message text: The target framework specific setting 'ProviderManifestTokenValue' hasn't been set to a valid value. This value is required by the Entity Framework at runtime. Source: Project Validation Created on: 04 February 2021 3:46 Corrections/suggestions Choice 1: Open the Project Settings and click the 'Entity Framework' node in the tree at the left. At the right, specify a value for the setting 'ProviderManifestTokenValue'

What should i write for "ProviderManifestTokenValue" if i am using devart dotconnect mysql 8.19 ?

Usually this is the database version, so you may try 8.19. This is an EntityFramework requirement and it is database-specific, so if you can't figure it out, I suggest you post on the DevArt forums, to see what they have to say about it.