First you have to set-up the bankaccount in Odoo properly.

It depends on fields of the bank statement download (csv).

You have to be sure about the account type and format of the accountnumber in the csv download.

If the normal bankaccountnumber is mentioned, not iban, than you have to set the bankaccounttype on Normal and you have to set the accountnumber on your normal number (not iban) in the bankaccount entry of odoo.

Download the statement from your bank en save it somewhere you like.

Then in odoo go to bankstatement, then press import and search for the csv you have saved. If you set up everything well, you should see the banktransactions