So many things...
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Secondly, don't ever create your database strings like that! Concatenating strings leaves you wide open to an accidental or delibetare SQL Injeection attack which could destroy your database. Use parametrized queries instead.
OleDbCommand oleDbCommand1 = new System.Data.OleDb.OleDbCommand("INSERT INTO user (email ,password) Values (@UN, @PW)", con);
oleDbCommand1.Parameters.AddWithValue("@UN", txtusername.Text);
oleDbCommand1.Parameters.AddWithValue("@PW", txtpassword.Text);
oleDbCommand1.ExecuteNonQuery();
Thirdly, never store passwords in clear text - it is a serious security risk. There is an explanation here:
Password Storage: How to do it.[
^]
"I had done it but same exception occurs"
Ah! It may be simple: enclose the name of your password field in square brackets:
OleDbCommand oleDbCommand1 = new System.Data.OleDb.OleDbCommand("INSERT INTO masterusertable (email ,password) Values (@UN, @PW)", con);
becomes
OleDbCommand oleDbCommand1 = new System.Data.OleDb.OleDbCommand("INSERT INTO masterusertable (email ,[password]) Values (@UN, @PW)", con);
"Password" is a reserved word in JET databases, that may apply to ACE as well.