RHEL 7 is supported, but it looks like the driver in 7.6 either doesn't like the device, or the firmware version. If you can, you might consider pulling the PCI card and completing the installation, then adding the card back in and installing the kernel modules. I'm not sure what your specific device is, but you should be able to check the support page to locate the current driver version we're hosting here:
If removing the card isn't possible, you may consider preparing an .ISO with the files and loading it to a USB or a web server. When the RHEL installer gets to the black screen where you can choose to begin the install or test the media, press tab. This will pull up a shell and you can append the inst.dd=driverLocation, driverLocation being a path to the driver update image you would need to prepare.
If you want to send over any other error info, I'm happy to look it over and see if there is anything I might be able to add.
RHEL 7.5 out of box driver will not work on RHEL 7.6 because non white-listed KABI used in driver. RHEL 7.6 has latest drivers so no need to use out of box driver