If you see the following error, it means Delta is unable to calculate the cost basis and hence the profit/loss. This occurs either because your holdings are dipping below 0 temporarily or because the cost basis is not known for coins deposited from source "Other / Unknown" or "Other Wallet".
You can find the transaction that makes holdings dip below zero by scrolling through the list of transactions and finding those with the following error.
Note that deposits from "Other / Unknown" or "Other Wallet" do not have a cost basis associated with them and thus do not contribute to holdings of Delta's cost basis calculator. Below are some examples of when you will get this error and how to correct it.
A deposit of 0.1 BTC from "Other / Unknown" followed by a Sell.
The deposit of 0.1 BTC does not have any cost associated with it so we cannot compute realized gains and cost basis correctly and you will see the error.
To remedy: add appropriate transactions to indicate where this 0.1 BTC came from. If it was bought elsewhere then transferred, then add the buy transaction and a withdrawal from that source.
A Buy of 1 ETH with proper cost basis but then a Sell of 2 ETH.
More amount is sold than is available leading to a negative balance. Here, Delta does not have the cost basis of the extra ETH sold and thus you will see the error.
To remedy: Correct the transactions so that your holdings don't dip below 0 at any point in time.
Example 3 (relevant only to Bitfinex users):
Bitfinex often deducts fees or amounts from coins more than available and then performs transactions known as settlements to correct this.
Thus, if you have connected to Bitfinex exchange connection, your coin balances may dip below 0 at times.
To remedy: Edit the settlement transactions' time so that it is before the trade that put the balance below 0.