AFAIK this is automatic, but there are a few reports of it not working on Pi.
Have a look in Settings / VRM Online Portal , scroll to the bottom
Free disk space is your SD card, but AFAIK this is not where data is logged when the VRM connection is down, because this is the OS drive - just a small amount of data (a few hours?) is stored in RAM.
“No storage found” means no additional USB or SD is available (the SD card that VenusOS is on can’t (or wont) be used by VenusOS to buffer logs - it has to be a USB port or SD slot, like you would find on a Cerbo, Cerbo-S, Ekrano etc. Pi doesn’t have a 2nd SD slot unless you add a hat, so USB is your only option.
So … I think you might have to populate one of your usb ports with a usb drive formatted as FAT.
RP>