You can only sell coins and currencies you own.
If the tax report detects sales without a suitable purchase, it will display a warning.
In this case, a purchase of 0 USD (or whatever your FIAT currency is) will be assumed, which increases your gain and therefore also the tax.

Please make sure that all your purchased coins and currencies are correctly and completely listed in CoinTracking.
Look at the detailed calculation in the "Overview of all created Tax Reports table" to see all sales with a warning.

Please note that deposits and withdrawals do not have a cost basis as they are considered movements between wallets or exchanges.
That's why they will not be calculated in the tax report.
If you have bought coins, you need to add them into CoinTracking as trade instead of a deposit.