This could be due to a number of reasons. Before contacting us directly, it is worth going through some of these possible reasons why you may be receiving this error:
1) Incorrect date format
2) Excessive row edits - e.g. editing existing transactions and adding new ones simultaneously. Try doing one type of edit first, saving it and then doing the second type
3) Incorrect data cell - e.g. a share price of $0 (it happens!)
4) Browser caching issue - you can clear your browser's cache and try again. We recommend using Google Chrome as the optimal browser for Simply Wall St
If error persist, you can also copy and paste the transactions of your current portfolio (the one which is experiencing an issue) into a new 'Transactions Portfolio' and save to see if the error occurs.