Why is the poet not clear about the type of sleep,he is about to drop into?
This depends upon how you interpret the poem.
Many people think that this poem is about the narrator facing death. In this interpretation, the apple picking symbolizes a long life that has been full of work, successes, and disappointments. Now, the narrator is going to fall asleep (die). He does not know what happens after death and that is why he is unclear.
I suppose you could interpret the poem more literally and say it is actually about a person who is tired. In this case, he would be unclear because he does not know if he will rest well given how tired he is.
Finally, I don't know if your question means what I just answered or if it means "how come the poet wrote it like this?"
If it means that, I would think Frost left it unclear because that is part of what makes poetry what it is. Poetry tends to leave questions like that for the reader to ponder.
In the poem 'After Apple-Picking' by Robert Frost the poet seems to have lost the will to enjoy picking any more apples! What was at thje start a joyful activity has become, by its very nature and duration, just a chore. The poet may have enjoyed and found fulfilment in the task at the beginning (like in life itself) but has run out of steam because it was too much for him and all he can think of is sleep. Perhaps he bit off more than he could chew, or perhaps he should have hired some help. We do know,however, from other poems, that Robert Frost was very particularly proud of his traditional orchards (Mending Wall.) The sleep he will get is unsure, because his bones are aching, his back hurts and every time he closes his eyes, all he can see are apples. This type of sleep disturbance is often a response to overwork or stress. As in 'The Path Not Taken' he leaves the ending as an enigma.