A phrase consisting of multiple words cannot have a single part of speech - part of speech refers to singular words in isolation. If you mean what the function of <em>on the couch </em>is, then the answer is - adverb phrase. If, however, you really do need a part of speech, then you have to do it word by word: <em>on </em>is a preposition, <em>the </em>is a determiner, and <em>couch </em>is a noun.
hey bro ,am writing this letter to inform you about my father's birthday party so will you be able to come cause his turning 60 which means all of his children should be there to celebrate with him and let him know that indeed we love him as his children and all we want for him is to be happy ❤️
