Cost basis on transactions conducted with WETH are set to 0.
Users will see an error highlighted in red that says:
Could not determine the value of this sale - we don't know the price of [asset] on [date] - please add the fiat value received or mark it as a transfer.
This problem appears often on NFT purchases using WETH, whereas purchases on the same platform (such as OpenSea) that use ETH are treated normally. This error is also seen on DeFi transactions involving WETH.
In order to make your data more accurate, you can manually modify these transactions as per this article on our help center. Please be aware that once we release a fix for this issue, you may have to undo the manual changes.
You can also try removing and re-adding the wallet, then check again to see if the cost basis on the WETH transactions populates correctly. Removing the wallet deletes all transactions and any manual edits, so please only remove the wallet if there are no manual edits you need to keep.