Zero-cost errors occur for a couple of reasons:
A simple rounding issue
A value becomes "0" when rounding occurs. This is often caused by a decimal point issue when data is exported from an exchange.
Fix: This can be rectified quickly with a check and re-upload of your CSV files.
More commonly, an acquisition is missing from the data you have imported into Recap. When our tax calculator recognises a disposal but cannot find a record of the cryptocurrency purchase it uses a cost value of "0" and flags the error.