SMB1 is disabled - which makes it necessary to specify the share as a part of the address.
The message is because you have not pointed to a folder - only to the server - so adding the share-name will likely do the trick.
On Plasma you may need to add the username to the address - for anonymous access you may be able to use guest - you may have to use the localized version matching your Windows system’s locale
For a more seamless experience create a designated folder structure
This is an example - amend to your use case (don’t be tempted to use /run, /home or /mnt)
sudo mkdir -p /a/documents /a/videos /a/pictures /a/work /a/projects
Make the subfolders writeable by everyone
sudo chmod ugo+rw /a -R
Then create 5 set’s of mount/automount units named after the tree pattern
Place the files in /etc/systemd/system
a-documents.mount and a-documents-automount
a-videos.mount and a-videos-automount
a-pictures.mount and a-pictures-automount
a-work.mount and a-work-automount
a-projects.mount and a-projects-automount
The content of a mount unit. Adjust the Where and What for each share - remembering the naming convention
/a/documents = a-documents.mount
a-documents.mount (observe property casing and no space before or after equal sign)
When you have created a mount unit and save the file - activate the unit and browse the path
sudo systemctl start a-documents.mount
Browse the folder and verify the content - then stop the unit.
Enable the automount unit. When you browse the folder the unit will activate the mount unit and the share is available. The share will be unmounted when the TimeoutIdleSec has been reached.
sudo systemctl enable --now a-documents.automount