HI, I think your field 'FK_catId' is int that why error could arise ,so you have to type cast the string value of combobox into integer like this below,
Convert.Toint32(comboBoxFK_catid.SelectedValue)
Full code below:
SqlCommand cmd2 = new SqlCommand("insertProducts_TBL", con2);
cmd2.CommandType = System.Data.CommandType.StoredProcedure;
cmd2.Parameters.AddWithValue("@Product_name", txtProduct_name.Text);
cmd2.Parameters.AddWithValue("@FK_catId", Convert.Toint32(comboBoxFK_catid.SelectedValue));