Problem
You are unable to edit the tax field to remove tax from an invoice.
Solution
Search for and go to the invoice.
On the invoice record, click Update invoice details.
Edit the tax field to 0.00.
Click Save.
If the Tax field is grayed out and you are unable to edit it:
Post the original invoice.
Create an adjustment invoice with a negative task and the same amount as the tax amount you want to remove.
Save the adjustment invoice.
The tax will now be removed from original invoice balance.