Try something like this in your code - string myQuery = "select * from" + tname + "where ...".

You can then use myQuery in your command.
Make sure to use command parameters to avoid sql injection.

Try this

string strSelect = string.Format("select * from {0} where ...", Label21.Text.ToString());

you should use
select * from ''+lblt.text.tostring()+" where <other>