When checking the summary on the "Tax Reports" page, you may see a "Review needed" section saying:
Review needed
We have assumed a cost of zero for some assets that were sold for $---.--. You can reduce your taxes by approx. $---.-- by fixing some issues in your wallets. You may also ignore this warning and submit your tax reports with the zero cost basis (this is allowed by tax authorities)
This message is related to "Missing purchase history" warnings on your transactions and means that some assets that were sold do not have a history of acquiring them - so we assumed $0.00 cost basis (acquired for free).
Clearing out those warnings may reduce your total taxable gains, as assets you sold would now have a cost of acquisition, resulting in lower capital gains upon disposal.
For more information about "Missing purchase history" warnings and help on fixing them, check the article: