Most probably you violate the foreign key constraint.
Your two SQL statements a little bit more clear:
insert into mydb.employee (idemployee,name,type)
insert into mydb.emp_account(idemp_account,loan,bill,balance,total)
values ( '" + this.textBox6.Text + "','" + this.textBox7.Text + "','" + this.textBox8.Text + "','" + this.textBox9.Text + "','" + this.textBox10.Text + "')
Where in the detail insert statement (emp_account) you have a foreign key to the master table?
At least this.textBox3.Text (I assume employee.idemployee is primary) should apear in detail insert to fullfill foreignkey constraint (with appropriate field of course).
It would help a lot to answer if you give a short overview about this two tables and there fields.