Digikam currently supports two database backends: sqlite and mysql. Unfortunately, digitaglinktree as shipped with digikam supports only sqlite currently. This is not very clear from the man page.
Digitaglinktree should be fixed to work with both backends. Ideally it should read the database information directly from the digikam ini file, so username and password does not have to be given on the command line. This would also make digitaglinktree easier to use for the sqlite case.
Another entry about digikamlinktree for you...
you can try digitaglinktree 1.8.4beta if mysql support therein works for you. You find this version attached to Bug 323507.
*** This bug has been marked as a duplicate of bug 323507 ***
Fixed with bug #323507