I don't feel well, I need to cancel.
'Annullare' — an-nul-LA-re. Double 'l' is clearly pronounced, stress on the third syllable.
Use when you are genuinely unwell or when you need a believable, sympathetic reason to cancel plans with friends or family. Less appropriate in formal professional settings.
'Non mi sento bene' uses the reflexive verb 'sentirsi' (to feel). 'Devo annullare' means 'I must cancel'. This combination is direct and elicits sympathy rather than disappointment from the other person.
Sto poco bene, non ce la faccio.
I'm not feeling great, I can't manage it.
'Non ce la faccio' is extremely colloquial — implies you genuinely cannot cope
Ho la febbre, purtroppo devo cancellare.
I have a fever, unfortunately I have to cancel.
More specific — inspires immediate sympathy and no pushback
Mi sono svegliato/a male stamattina.
I woke up feeling bad this morning.
Casual, used among close friends — implies you are not at your best
Italians take illness seriously as a reason to cancel. It's also common to receive unsolicited home remedy advice — be prepared for suggestions involving chamomile tea or a hot broth.