Thank you all for your feedback. Much appreciated.
Actually, we realized that the 2016 Quickbooks file was in Windows format and therefore, it needed to be converted to a Mac format (qqb).
Once we did this, we managed to open the Company File from Quickbooks 2019 without difficulty and what is more, Quickbooks no longer asked us to input a password.
It should be noted that Quickbooks 2019 did not automatically ask us to update/upload our company file from the previous Quickbooks 2016 version. In our view, it should have been the case in order to ensure a smooth transition between the two versions.