Most likely is because that the realtime clock (RTC) on your printer is not current. The ZD421 does not have the battery. The RTC may got reset after power off. Please set the RTC to the current local time with the following commands. When the RTC is off, the TLS authentication will fail and will result in the printer's Weblink connection fails. If you want to restore the RTC automatically after a power off, you can enable the NTP (Network Time Protocol) on the ZD421 with the following commands.

### Set the RTC to local time
### Change the values to your local date & time
{}{
"rtc.time": "10:53:00",
"rtc.date": "08-02-2023"
}

### Enable the NTP to point to a NTP server
{}{
"ip.ntp.enable": "on",
"ip.ntp.servers": "0.us.pool.ntp.org"
}

I think you can try verifying that the file you are sending is in a supported format for your ZD421 printer. The Zebra Programming Language (ZPL) is the native language used by most Zebra printers, so make sure your file is in that format.

I follow the Free PDF Direct with Link-OS 6.3 but it does not work after that.

I have done:
- Application is created and validated
- My devise (ZD421) is correctly registered on Zebra
- registered into SendFileToPrinter(FREE)
- done the 2 zpl command for the time
- done the PDF simulation

What is missing ? It’s been one week that I have called the Savanna Support and I’m waiting that they called me back to find a solution...