Romance languages generally do not have a way to express "do" as it is used in this context. In English the original verb and location is implied by saying "I do" ("I do sleep on the sofa"), in Portuguese you can't say "eu faço" but instead say "eu durmo" ("eu durmo no sofá").
Yes, "durmo" is in the simple present tense (I sleep). It is irregular though, and rare. Similar verbs are "tossir" (to cough) and "cobrir" (to cover). "Eu tusso" and "eu cubro".
Sometimes duolingo requires litteral translations and sometime not. "but I sleep" is not accepted. Ok, but not consistent with translation requirements for other sentences.
It has nothing to do with how English speakers think. You have to understand how to use prepositions. "In the sofa" does not mean "on the sofa". They are very different.