Alma 12

As Alma taught the plan of salvation to the people of Ammonihah, and more specifically in answer to Antionah, he spoke of the story of Adam and Eve and the Tree of Life, saying -

"...I say unto you that if it had been possible for Adam to have partaken of the fruit of the tree of life at that time, there would have been no death, and the word would have been void, making God a liar, for he said: If thou eat thou shalt surely die."  (Book of Mormon | Alma 12:23)

Adding -

"...behold, if it were possible that our first parents could have gone forth and partaken of the tree of life they would have been forever miserable, having no preparatory state; and thus the plan of redemption would have been frustrated, and the word of God would have been void, taking none effect."   (Book of Mormon | Alma 12:26)

I want to specifically note two things.  Adam and Eve were told the consequence of their agency or choices.  They were told that if they chose to partake of the forbidden they would die.  Thus the consequence was that they did die and were not allowed (if possible and it wasn't) to have instant immortality.

The second point is that we are taught that we are here to experience agency and choice, find happiness or misery, and to progress or grow thereby finding joy and becoming more like our Father in Heaven.  Alma teaches that if Adam and Eve were not allowed to live in mortality in die, they would be miserable forever for the same reasons mentioned above.