filesystem error: cannot create directories: Permission denied [/mnt/hdd/bitcoin/regtest/wallets]
bitcoin in AppInit()
Try to add -datadir=
Take note that the specified directory should be existing prior on using the command,
also, the specified datadir= is the "bitcoin" datadir so instead there, a "regtest" folder will automatically be created inside it where the regtest files will be created.
The OP mentioned that it's a raspiblitz setup, IIRC it runs bitcoind as it's own user so you will have to make sure that that user can write to that directory, not the user you SSH in as.
I like the pre-done nodes like this a lot, but you are living in their world to a certain point so things that might be simple on a device you did yourself might require a little more tweaking on one of these.
-Dave