Basically, both acá and aqui refer to something close to the speaker. That much is clear. However, my sources appear to conflict a bit as to how they are used. Some say acá is used with motion verbs and aqui is used with non-motion verbs. Other sources say acá is used in a more general sense of here while aqui is specifically this spot. Not sure which is right or if this is a regional thing.
Running, walking, swimming, anything that implies movement. Non-motion verbs would be like being, looking, feeling, etc...
The adverbs for distance vary quite a bit throughout the Spanish-speaking world. Here is the link to the Butterfly episode on the topic:
If you Google it, you'll find a huge number of conflicting opinions, pertaining to usage and region. If you're looking to speak a specific version of Spanish, I'd try and learn the subtleties for that dialect and simply stick with them. Be wary of any "right answers" to this problem, as native speakers from different parts of the same country seem to have completely different opinions on the matter.
They effectively mean the same thing, but daily tends to be used to denote a scheduled occurrence (we have a daily meeting), whereas "every day" does not. Note, don't confuse this with "everyday" which means casual/basic/usual.
Also, your question. Should be "why can't I say..." or "why can I not say...". "Why cannot I say" is technically gramatical, but it's not a phrase anyone would use outside of poetry or other heavily stylized communication.
How would I know if it is "can you come" or "do you come"? All of the previous questions were "can".
In my opinion 'can you come' would be 'puedes venir'. Not sure about what previous questions you are talking about but I have seen some newer questions where Duo adds the 'can' at the beginning. Most of these were really 'may' questions (which Duo often rejects as an answer) like 'May/Can I help you?' (Puedo ayudarlo?). In this case 'May you come here often? sounds a little strange so I think it would have to be 'Do you...'
"Do you come here all the days" should be accepted too, right? Todos also means all.
'All the days' would be a very uncommon phrase and would only be said 'every day'
Aquí is used all over Latin America. It is not limited to Spain. Aca is often used for motion. I've heard Spaniards say "venga aca".
I used "can" instead of "do" because other sentences have been doing without the "Poder" and I was marked wrong. Why?
Why was my answer "Can you come here everyday? was marked wrong? There is no indicator whether you should use DO instead of CAN before the verb.
I think you were down voted because this has already been answered above. Can you come would be 'puedes venir'.
Not exactly as it isn't 'acá' that is informal it is 'vienes'. (tú) vienes is informal and (usted) viene is formal. As for 'acá' that has been aswered in a couple of previous posts. Please reference them.