ASP.NET application with Oracle Database and Entity Framework 6 - Compatible Entity Framework database provider couldn't be found

I'm getting this error.

Your project references the latest version of Entity Framework; however, an Entity Framework database provider compatible with this version could not be found for your data connection. If you have already installed a compatible provider, ensure you have rebuilt your project before performing this action. Otherwise, exit this wizard, install a compatible provider, and rebuild your project before performing this action.

Update: The project multiple layers based on onion architecture. I have realised that I am able to add new model to other solutions / layers. However I'm not able to add this one which I keep most of my models.

My system properties:

• I'm running windows 10 x64 Visual Studio Enterprise 2015 x64
• Target framework: .NET Framework 4.5.2
• Entity Framework 6.1.1
• Oracle.Managed Data Access Entity Framework 12.1.24
• Oracle.ManagedDataAccess 12.1.24160
• (ODT) Oracle Developer Tools for Visual Studio 2015
• ODAC 12.2c Release 1 64-bit (12.2.0.1.0)

What I have tried

1. I have modified entity framework string like this

 <provider invariantName="Oracle.ManagedDataAccess.Client" type="Oracle.ManagedDataAccess.EntityFramework.EFOracleProviderServices, Oracle.ManagedDataAccess.EntityFramework, Version=6.122.1.0, Culture=neutral, PublicKeyToken=89b483f429c47342" />
 

    <defaultConnectionFactory type="Oracle.ManagedDataAccess.EntityFramework.OracleConnectionFactory, Oracle.ManagedDataAccess.EntityFramework">
</defaultConnectionFactory>

2. I have uninstall both ODT and ODAC and installed and installed previous release of them.

3. I ve downgraded Entity Framework from 6.1.3 to 6.0.

4. I have change project build option from "Any CPU" to "32 bit" and tried installed all the components accordingly in 32 bit. Didn't work.

5. I have checked or compared web config and app config files which contains any configuration about oracle and entity framework, but couldn't see anything wrong.