Can I guarantee seeing the northern lights?

The answer

No. The aurora depends on solar activity and on clear, dark skies, and no one can promise a sighting, however far north you go. Cloud cover can hide the sky entirely, even on an active night. The honest approach is to stay several nights in the north, choose a dark place with a clear view of the northern sky, dress warmly and treat the lights as a bonus. Be sceptical of any source that claims certainty.