Executable has network-related or instance-specific error

I built in executable in Visual Studio that serves as an application for users to make changes to data in a SQL Server. When I run the executable on my machine it works fine. Another developer is also able to run it on his, but whenever we send it to users to run they are hit with:

"System.Data.SqlClient.SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)"

Have been at my wits end trying different solutions with nothing working. Does anyone have any advice?

Thanks.