I'm getting an error when importing my Coinbase transactions. "We weren't able to calculate some of your cost basis." However there are no transactions that need review.

Depending on the number of transactions you have imported, one solution would be to look at any that have a zero cost or sales amount and enter 1 in both cost and sales boxes.  If both are showing zero you can delete the transaction with the trash can icon.

If some have sales amounts but no cost basis and some have basis with no sales amounts, keep track of them, add all the sales amounts together, and all the cost basis amounts together, and combine them into a single transaction outside of the import feature.  After this, delete the entries you combined.  This has worked in the past.

Another method would be to request if you do not already have one, Form 8949 from Coinbase.  Delete all imported transactions from Coinbase, and enter the totals as a single transaction as shown on the 8949.  

The IRS is concerned that you are properly reporting your total capital transactions with their associated cost basis and while individual transactions are important, the totals are the most important.