Try removing tax from the transaction and see if it saves. If it does save, then the problem is due to the tax item setup. Go to Lists, Tax Item List, Open the tax item you were using - eg: GST - with out clicking on anything, look at what name appears in the Tax Agency field and take note of the exact spelling (with out touching anything yet). Now that you have the exact spelling of the tax agency, click on the drop down box and see what it shows, if it shows something different, even slightly different for example (Australian Tax Office vs Australian Taxation Office) then choose what it shows in the drop down box and save. Repeat for all your tax items.
You should then be able save your transactions with GST.