Translation:Please pour some sugar into my coffee.
That's a grammatical sentence, but means something else. Whenever you have the preposition en, you need to make the noun phrase after it accusative if you want the 'into' meaning.
Bonvolu ŝuti iom da sukero en mia kafo. = 'Please pour some sugar (while you're) in my coffee.' (Not specified what you'll be pouring the sugar into.)
Bonvolu ŝuti iom da sukero en mian kafon. = 'Please pour some sugar into my coffee.'
Of course, the en mia kafo version describes an extremely implausible request. Here's a pair where both meanings are very plausible:
La infanoj kuras en la domo. = 'The kids run (and they are) in the house.' (They're in the house the whole time they're running; they don't come in from outside.)
La infanoj kuras en la domon. = 'The kids run into the house.'
You would have to change "Bonvolu" to "Bonvole". "Bonvole ŝutu iom da sukero en mian kafon" would be OK. You can't have two imperatives in the same sentence, except in a sentence telling someone two do more than one thing, such as, "Go to your room and tidy it!".
Although word-for-word, the English translation is correct, we don't usually use "pour" for small amounts. We'd say "Please put some sugar into my coffee." I think this is one of those instances where it is better to translate the meaning, rather than the individual words, but I must admit I have not tried using "put" here.
Because there are many natural languages that use an accusative for direction (e.g. latin, Hebrew, German) Using an other letter only for ditection would have been an option and it might even have been easier for you and maybe for other native English speakers and people who are only familiar with languages that do not have a accusative of direction, and especially for native speakers of a language that has a special case to expres movement in a direction (e.g. Finnish, Hungarian and Estonian have some locative cases for different movements: an illative case just for movement into something, an elative for movement out of something, an allative for movement 'onto' and an ablative for mivement away from something), but is would have been an extra hurdle for people with another linguistic background. Keep in mind that esperanto in some way is always a compromise. And speaking for myself I am glad esperanto only has two cases.
-N has two meanings: accusative and marker of deleted preposition (the course creator should have said "don't use the accusative -n after prepositions" but that would be too confusing for beginners, i think)
If you remove a preposition, you add an "n": -ŝuti sukeron al en la kafo (=sukeron en la kafon)
-en lundo mi naĝos (=lundon mi naĝos) (=monday i'll swim)
-mi iras al londono (mi iras londonon)
The section notes explain why this is, but in case you haven't read them, here they are:
THE DIRECTIONAL -N
In addition to its use for the direct object, the -n ending is also used to show direction:
Ŝi saltas sur la tablo. = She jumps (up and down) on the table. Ŝi saltas sur la tablon. = She jumps onto the table (from another location).
I think "Please put some sugar in my coffee" would be the most natural English expression, but I haven't tried it here, so I don't know if it would be accepted.
Woe is me! I suppose it I was asking for too much when I hoped that Esperanto wouldn't do exactly what distresses me most about English: Using one suffix for three different meanings. Just as English adds an S (or 'S - there is no difference in sound) to the end of words to imply either 1)possession (with exceptions) 2)plural (with exceptions) or 3)contracted "is"; Esperanto now uses the N suffix for 1)direct object (with exceptions) 2)directional movement and 3)replace prepositions. Sigh... I can cut my disappointment with a knife...
I also had hoped for a perfect language, with entirely regular everything. However, it is less messy than English and usually (but not always) it is more logical.