I found that the tax code fields but the active flag, can not be edited in 3.1.7 after the new tax code line has been saved.
Once a tax code is created, it can not be edited after it is savedother than the active status and its name. If your tax code percentage has changed, create a new tax code and set to active, and set the old to inactive.
Do also check out many topics regarding this by searching this forum. The Search menu option on the top of this forum allows you to enter in a search term, which can be useful to identifying if someone else has posted a similar question or issue.
For your convenience, I do highly recommend referring to the AyaNova Help docs for details on any feature in AyaNova. The AyaNova Help docs section https://www.ayanova.com/AyaNova7webHelp/index.html?tax_codes.htm includes information on tax codes, and what can and can not be edited.