BACKUP LOG cannot be performed because there is no current database backup
Originally, I created a database and then restored the backup file to my new empty database:
Right click on Databases > Restore Database > General : Device: [the path of back up file] → OK
This was wrong. I shouldn't have first created the database.
Now, instead, I do this:
Right click on Databases > Restore Database > General : Device: [the path of back up file] → OK
Another cause of this issue is when the Take tail-log backup before restore
"Options" setting is enabled.
On the "Options" tab, Disable/uncheck Take tail-log backup before restore
before restoring to a database that doesn't yet exist.