Assuming you bought Magix Video Pro X, direct from Magix did you copy and paste the serial number, including the P3- , from the purchase confirmation email into the activation dialog, if not try this.
If you did not buy direct from Magix, then you may have a coupon/voucher code consisting of a sequence of letters and numbers that has to be redeemed first. To redeem the voucher code do so here, you will then be emailed a serial number.
If neither of the above and you are getting an error code/message:
-14 generally means that you're putting a valid MAGIX serial number into the wrong MAGIX product or version/edition
-24 serial number has already been registered to another email address - contact support - then use the same email address as you purchased Video Pro X with
If you purchased Vegas Pro then this is the wrong forum - the Vegas products have their own forum see the Important Posts here.