Did you happen to create the mount point first?
Code:
sudo mkdir /mnt/test_data
systemd-mountd will create directories as needed (/etc/fstab or a manually created unit-file), but when you manually use sudo mount, the admin has the responsibility to ensure the mountpoint actually exists.
Try using the IP address instead of the hostname. If you are running avahi, you can try using winsys1.local, but I don't know if that works.
There's also a question of SMB/CIFS versions matching. If you have other CIFS shares from the same server working connected to this Ubuntu system, then that probably isn't the issue.
Not that this matters, but I generally use these mount options to mount my last MS-Windows system (Win7),
Code:
iocharset=utf8,rw,vers=2.1,uid=tf,gid=tf,file_mode=0664,dir_mode=0775,credentials=/etc/samba/win7lap-D.credentials ://172.22.22.14/K
I think Win10+ support vers=3 as the minimum. Better security and performance. Also, rather than putting the username/password in the mount command, it is best to place those into a "credentials" file. Anything in a command line can be viewed by any user on the system. Best not to post credentials to other computers for all to see.
Those are all the ideas I have. Try them in order. That's what I think is most likely.