In order:
1) use a 16gb pendrive, FAT32 formatted (NTFS or other filesystem will not work) and replay all the process. Disable antivirus software on your PC before starting, (even Microsoft Defender or other Microsoft antivirus) some files could be deleted by Windows if false positive. Check the amount of files uploaded on your pendrive once the Garmin Express PC app ends the process, it must be higher than 8gb.
2) Try rebooting your unit: press and hold the Power and Day Night button at the same time for five seconds. After five seconds the unit will switch into the reboot mode, then the unit will do a file check during rebooting. Rebooting will take approximately two and a half minutes, then replay step 1).
3) If step 2 does not work: try a Factory data reset: Go to the HOME screen, press the "Apps" icon, press "Settings" icon, select "Backup & Restore" then "Factory data reset" then "Reset device" then "Erase everything" and finally conform. This will erase all your personal and stored data such as paired phones, Wi-Fi, phone books, uploaded pictures, FM and DAB stations, etc. Then replay step 1.
4) the software version of your Garmin App is not the last available I have the 3.45 corresponding to the RC8, the last release from Honda, your one could be a RC5 or RC6 and an update could be a good idea.