MySqlCommand Command.Parameters.Add is obsolete

try AddWithValue

command.Parameters.AddWithValue("@mcUserName", mcUserNameNew);
command.Parameters.AddWithValue("@mcUserPass", mcUserPassNew);
command.Parameters.AddWithValue("@twUserName", twUserNameNew);
command.Parameters.AddWithValue("@twUserPass", twUserPassNew);

and don't wrap the placeholders with single quotes.

string SQL = "INSERT INTO `twMCUserDB` (`mc_userName`, `mc_userPass`, `tw_userName`, `tw_userPass`) VALUES (@mcUserName, @mcUserPass, @twUserName, @twUserPass)";

Edit: As Bradley Grainger pointed out, in MySQL is safe to use AddWithValue. I'm keeping my answer if you get here by chance and use Microsoft SQL.

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Please read this article, advising you against using AddWithValue:

https://blogs.msmvps.com/jcoehoorn/blog/2014/05/12/can-we-stop-using-addwithvalue-already/

It says basically that AddWithValue could sometimes incorrectly infer the correct type. Use Add instead.

Just edit/remove some code in this part

('@mcUserName', '@mcUserPass', '@twUserName', '@twUserPass')

to

(@mcUserName, @mcUserPass, @twUserName, @twUserPass)

and Add( to AddWithValue(