Do I need to pay VAT for unrealized sales? How can I bind a business tax account bound by someo

I registered for VAT (annual accounting scheme) on April 1st, but three months have passed and my website has no sales income, so I cannot pay the VAT tax of 500 pounds that is due at the end of July. Please tell me what should I do in this situation? At the same time, I logged into the UK Government gateway. I used this gateway ID to apply for UK VAT registration at that time, but now when I want to bind the business tax account of this company to this gateway ID, it prompts me that the business tax account of this company has been bound to another gateway ID. I don’t know who did it. So how can I bind my own company’s business tax account back to my own Government gateway ID?

You need to write to HMRC and ask them to consider a lower amount (the amount cannot be Nil) - see sections 4 and 8 here Annual accounting (VAT Notice 732).
You have probably chosen the wrong scheme to be on if you are a new business with uncertain sales, you may want to consider moving to normal quarterly returns as if there are no sales in a quarter then you can file a Nil VAT return and nothing to pay.
The government gateway (12 digit User ID and password) only allows ONE VAT service to be attached to it, so if the system is saying there is already a VAT number attached to it, then that is either a technical issue with HMRC or you are using someone elses gateway/login ID. For gateway issues, you need to call the VAT IT helpdesk using the telephone number on this link Get help using VAT online services  or you could write to HMRC but this will take longer to resolve.
When calling the IT helpdesk you will be asked several security questions about the business so make make sure you have your VAT details and information in front of you.

Hi Flora,
You would have to speak to our Annual Accounting Team as regards the interim payments of £500 which you are due to make. 
Please contact our Helpline on 0300 200 3700 who will be able to redirect you to the Annual Accounting team to discuss the issue.
As regards the online issue please speak to our Online Services Helpdesk on 0300 200 3600 who will be able to resolve this for you. Details are also at Technical support with HMRC online services
Jay thank you for trying to assist with your helpful post. I have changed the HMRC website details to hyperlinks.
Thank you