If your VAT number is showing as invalid when you input this into the system, there are potentially a few reasons for this.
The best thing to do first would be to check the number using HMRC's online checker here. This will confirm whether your number is valid.
The best place to get your VAT number from is from your VAT certificate, that is issued by HMRC. If you have an online government gateway account, and have linked your VAT account in the past, you should be able to get a copy of this here.
If you have only recently registered for VAT, there may be a delay in the number being recognised. In this situation, it is best to wait a few days and then check back to see if there is still an issue.