Sorry, your question is not very clear to me. Are you upgrading from 17.3 to 19.4 and getting error to perform firmware upgrade? If yes, what is the issue that you are facing with firmware upgrade.
Can you also let me know the chassis model, RE model, exact current version, and current RE i40e-NVM firmware version so that we can glean out relevant information from the following public KB.
Use following commands to get all the info
show chassis hardware | match "chassis|Routing Engine"show system firmware | match "Routing Engine"show version | match "Junos:"