How can enable Tax setting during the time of entering the bills ?

Let me share a solution on how you can enable Tax setting during the time of entering the bills in QuickBooks Online (QBO). 

It looks like the VAT feature isn't turned on yet, which is why you're unable to attach the VAT amount with the fees. Here's how to turn it on: 

1. Go to Taxes, and select Set up tax.
2. Set up the tax agency details such as the name of your tax agency, any official tax number that has been assigned to your company, your reporting method, how often you file your tax to your tax agency, and whether this tax is meant for sales or purchases, then select Next.
3. Follow the steps to finish the setup and select Ok.

To learn more about using the tax feature, check out this article: How do I set up sales GST/VAT rates and use them on forms? (International QBO).

After setting it up, you can start assigning VAT on your bills. On the Bill creation page, simply select Exclusive of Tax or Inclusive of Tax under Amounts are. Then, choose a rate under the TAX column. 

You can also browse other help articles here. I'm sure they'll guide you in managing your transactions and financials in QuickBooks Online. 

That's it! Please let me know how it goes or if I can be of additional help. I'd be glad to answer should you have any follow-up questions. Take care always.