"Unfortunately, I cannot come."
Translation:Jeg kan dessverre ikke komme.
Ei and ikke are interchangeable, although ei is slightly old fashioned.
Regarding the structure of the sentence, You need ikke between the verbs, as in the English sentence. Can not come - kan ikke komme. "Unfortunately" can in Norwegian fit in between the modal verb and the main verb. Dessverre, jeg kan ikke komme. Jeg kan dessverre ikke komme. Dessverre kan jeg ikke komme. Jeg kan ikke komme, dessverre. If this was present, I am not coming = Jeg kommer ikke.
To my understanding, the comma forces a new clause, thus forcing the standard "rule" of the verb following the subject. To avoid your issue in other, more advanced sentences, I'd assume reordering the words to fit the grammar rules be necessary.
"Dessverre, jeg kan ikke kom til parti dine, men jeg skal prøve å kommer en annen tid. Kanskje vi vil legge en plan å se hverandre snart!"
Apologies if my grammar/vocabulary is wrong, its 2:30 AM and I'm still a beginner, somewhat.
"No split infinitives" is a 19th century prescriptivist "rule" that has no more real force than "you can't end a sentence with a preposition" (or "a preposition is something you can't end a sentence with"). I could certainly imagine some English lord saying "I can, unfortunately, not come" (same word ordering). It's just more natural in the default translation, that's all.