Translation:You should log in to the instant messenger.
It is possible to say “al la tujmesaĝilo” and even “je la tujmesaĝilo” but "al" and “je” do not take an accusative of direction. "al" does not need it because it expresses direction by itself; “je” does not take it because it does not express a location.
The fact that “(en)saluti” is transitive does not mean that you cannot replace the accusative with a preposition.
AFAIK you express logging in into something by ensaluti en la tujmesaĝilon. That is with metamorphic movement into a system. I couldn't find any hits in Tekstaro for ensalut/, but when I googled for it (hint: I used Kukolo), all I could find was either with en + -n or some single cases of ĉe (if any object at all). None with a direct object.
While you can argue, that any preposition can be replaced with an accusative (la regulo numero 14), this has changed over time and in contemporary use you don't have so much freedom any longer.
Sorry, according to the 16 rules from Wikipedia not any preposition can be replaced with the accusative but just the preposition “je.”
14 • Ĉiu prepozicio havas difinitan kaj konstantan signifon; sed se ni devas uzi ian prepozicion kaj la rekta senco ne montras al ni, kian nome prepozicion ni devas preni, tiam ni uzas la prepozicion je, kiu memstaran signifon ne havas. Anstataŭ la prepozicio je oni povas ankaŭ uzi la akuzativon sen prepozicio. [emphasis by me]
By all means – but rule no. 14 simply does not say that “any preposition can be replaced with an accusative,” and such rumours are almost impossible to control on Duolingo. Just look how many posts claim that “word order does not matter in Esperanto.”
What shall we do when a learner translates “the chair is between the windows” by “
la seĝo estas la fenestrojn,” claiming that “inter” can be replaced by an accusative?
I find this to be inconsistent with another example which forced me to say something like "elsaluti el la babilejo" (it might've been tujmesaĝilo. And maybe there was an accusative ending, maybe not. I'm not here to contest that aspect.)
Can someone please clear the air on proper grammar for ensaluti+object and elsaluti+object?
I think you're on the right track - and I would not give a lot of credence to the other explanations in this thread which suggest that ensaluti is transitive. It seems to me that either they both (i.e. both ensaluti and elsaluti) are transitive, or that something else is going on.
I do not have a ready analysis for these two particular words - and as has been said elsewhere in this thread, examples from good authors are hard to come by -- but I have looked at the pair eliri and eniri -- and off the top of my head, I would suggest that they work in a similar way.
So, if it's possible to say elsaluti el la babilejo, I would expect the course to accept ensaluti en la babilejon.
It's also true that the accusative is often used to show the target of movement. This is sometimes used to explain why you say:
- eniri la buson
- eliri el la buso
And I don't see why the same logic couldn't apply here.