Having trouble logging into Filezilla under the 'root' account.

Since you are using ssh credentials to login to Filezilla, I would ensure that the 'root' account has permission to login via ssh. To verify this, you can run the following command and verify the "PermitRootLogin" setting:

egrep -i '(permit)' /etc/ssh/sshd_config

If this is set to "no", you can change this to "yes" by editing the file.

vi /etc/ssh/sshd_config

I would consider leaving this set to "no" rather than changing it, primarily for securtiy reasoning. The other option would be to create a normal Linux user account and connect to Filezilla using this account.

https://www.linode.com/docs/tools-reference/linux-users-and-groups/#creating-and-deleting-user-accounts