Could you say כי אין לי or is the reverse better
...because I haven't. Is it also correct?
I have never heard כי לי אין only כי אין לי is this a normal way to say it?
Well, I think it adds emphasis: as for me, I do not have one.
It didn't accept "because I don't" when I tried it. Reported!
They are accepting it now.
I thought I learned that כי shouldn't begin a sentence, unlike near-synonyms מפני ש and משום ש. Is this correct?
Well, the כִּי begins a main clause here: כִּי אֵין לִי For I do not have one (The difference would be more obvious in German because of word order: Hat er einen Schlüssel? Denn ich habe keinen. Not weil ich keinen habe).
Listen which languages from the key words can time
האזן אילו שפות ממילות המפתח יכולות לזמן
Yesh lo mafteakh? Ki li ein.