I do not understand why it happens, because if I enter the class inside the app_code folder on the website there is no problem, I'm new with c # and mysql I'm sorry if it's silly, on the other hand, the connection string is it okay ?, thanks:)
Pd: it is visual studio 2017.
As I see it, you are doing a project of the type Aplicacion Universal where DataTable/ Dataset/ DataViews no longer exist within the namespace System.Data , that's why you do not recognize it.
To continue using MySql the first solution would be to use the MySql.Data.RT namespace that would allow you to run querys. Solution Link
The other solution would be to use Connector.NET 6.9 from mysql to run your queries. Solution Link
Source: SO thread , System.Data