I try to import my Csv file to Chart of Account, I have error message, “The account number you have chosen is already used.”, because i need to update my account information, so i need to do that. I checked update and add , but i have error, accountnumber cannot duplicate? but other values are updated correctly. please tell me the reason.

Account numbers must be unique. One/some of the accounts that you're importing has/have account number/s that is/are already existing in your COA.

that account number does not include external id, so i need to edit and add external id

Why don't you just edit the duplicate account number/s in the UI before importing if the duplicates are only a handful? But yeah, you can add External ID. Just add an additional column for that but that won't solve the issue on duplicate account number/s.

yeah if you want Netsuite to reliably update an existing record, you need to reference either an internal ID or an external ID. What we do is we use the account number as our external ID. So when uploading initially we map the "account number" field twice: once to number, and once to external ID