How to import transactions when Quickbooks is older than three years?

Once Quickbooks becomes older than three years (currently Quickbooks 2019 or earlier), it stops importing QBO files and stops direct transactions download from online banking (bank feeds).

What can you do in this case? If upgrading Quickbooks is not an option, you can import transactions in the IIF format.

The ProperConvert app works similar converting to QBO (web connect) or to IIF files:

start the app and load your file with transactions (QBO, QFX, OFX, QIF, CSV, PDF)
click on the IIF tab ont he right sidebar
set the bank account name (notice, it is not an account id as for QBO files)
set the account type
use the Renamings tab to rename payee names to match vendor names in Quickbooks
check that the category column has expense/income account names
use the Renamings tab to assign categories (expense/income account names) based on payee/vendor names
for subaccount, specify full account path like ParentAccount:Account (this applies to bank/credit card account names and to expense/income account names)
verify that the category column has values for all transactions (in case this column is empty, default expense or income account name will be used (see the Settings-IIF tab)
make sure to backup your Quickbooks company data file before importing any IIF file
use the legacy method to import IIF is you would account names and vendor names created on fly
for any IIF import errors in Quickbooks, look for a new IIF file with the same name and added timestamp - open the file in a text editor, correct errors in Quickbooks and reimport timestamp file (which has only failed transactions)