unable to import OleDbConnection in dotnetcore entity framework while transferring data from excel to sqlserver
OleDb is not available in .NET Core, probably because it's not cross platform.
Have a look at the System.Data.OleDb Namespace in .NET Core 2.0.
Alternative
For a decent alternative, have a look at the answer to this Stack Overflow question: How to get OleDb for reading excel in asp.net core project
OleDb is now available for .NET Core. System.Data.OleDb is available on NuGet.org