I'll provide you with detailed steps when keeping track of taxes on your purchases, usersasamoks.
You'll need to set up VAT on purchases first to track these taxes on your transactions. To do this:
Go to Taxes, and select VAT.
Select the Edit VAT ▼ drop-down arrow, then select Edit rates.
If VAT for Purchases has not already been added, select Add tax rate.
Select the Purchases checkbox.
Give the new tax rate a name, description, and percentage rate (usually the standard), then choose an account and determine which return line you want the tax amount to appear on.
If you already had created entries without the taxes added, you'll have to modify the invoices, add the specific taxes, and re-send them again to your customers. This way, they'll receive the entry with sales taxes on it.
However, if these transactions were already paid, you can create a sales entry for the adjustments of the taxes that weren't added to the previous purchases. I recommend consulting your accounting professional for further guidance.
I'm also including this reference for additional input: Set up and record purchase taxes. Though this resource is specifically for Ireland, the information and processes are the same.