Long ago there was a default user name and password: “nvidia”. The documentation you are speaking of would be from an earlier time. Use “peto” and the password you created.
For developer kits using SD card to boot with you should know that some or all boot content (depending on release) goes into QSPI memory on the Jetson itself. That content must be compatible with the SD card content. QSPI does not change very often, and so many SD card releases will typically work without flashing QSPI. However, mixing SD and QSPI release can result in the Jetson trying to update the SD and changing it.
You will probably need to use an Ubuntu 18.04 host PC to flash QSPI, and possibly the SD card will then work. Or you might need to create a new SD card install. JetPack/SDK Manager is how you update the QSPI.
You might also find it useful to know that recovery mode is incapable of asking for a password. If it reaches this stage, then it means it rebooted into Linux and it thinks this is first boot and needs account setup.