I have added my tax % code in my settings.
When I am attempting to create a purchase invoice, I have tried using the tax drop down menu at the end of each line to add tax, but when I select the appropriate code it will still not add the tax to the total.
Can you post screenshots of both output and edit windows
Frequently, you can just enter the tax-inclusive amount in a purchase invoice. There is a tick box at the upper right corner of the entry screen to indicate this. You did not provide any specifics on your tax jurisdiction, but tax codes are generally related to taxes added to sales. The tax amount goes automatically to the Tax payable account. So the reason to enter a tax code on a purchase invoice is when the tax will actually offset the tax your must remit to the tax authority.
Again, since I don’t know your specific situation, I can only offer general thoughts.
You should be aware, regardless, that selecting a tax code on the entry screen does not produce any visible result. The result appears when you view the finished purchase invoice because tax is calculated on the total of items subject to a given tax, not on individual items.