It is a bad idea to put spaces in table and column names. If you do that, you have to enclose the names in square brackets to make sure the name is considered with the spaces:
INSERT INTO [SFA Inventory] (IMEINo, ProductName, Supplier, InvoiceNo, InvoiceDate, MobileNo, AppVersion) VALUES (@IMEINo, @ProductName, @Supplier, @InvoiceNo, @InvoiceDate, @MobileNo, @AppVersion)
Also, remove all of the spaces in your named parameters:
sqlquery.Parameters.AddWithValue("@IMEINo", TextBox2.Text)
sqlquery.Parameters.AddWithValue("@ProductName", TextBox1.Text)
sqlquery.Parameters.AddWithValue("@Supplier", ComboBox1.Text)
sqlquery.Parameters.AddWithValue("@InvoiceNo", TextBox3.Text)
sqlquery.Parameters.AddWithValue("@InvoiceDate", DateTimePicker1.Value.ToShortDateString())
sqlquery.Parameters.AddWithValue("@MobileNo", TextBox4.Text)
sqlquery.Parameters.AddWithValue("@AppVersion", TextBox5.Text)
You're also make another grievous mistake. NEVER directly use the values of textboxes in your queries. ALWAYS validate the data that's entered and use the validated data in the parameters.