I have already said why not several times. I'll try again, using different wording. When we look at any individual bible verse, it is essential to take note of the context (the surrounding verses). Take a verse like this:
“Then He said to them, "But now, he who has a money bag, let him take [it], and likewise a knapsack; and he who has no sword, let him sell his garment and buy one.” (Lu 22:36 NKJV)
Out of context, we could wrongly assume that Christians with no sword should sell their clothes in order to buy one! Similarly with the verse that started our discussion:
“Your sun shall no longer go down, Nor shall your moon withdraw itself; For the LORD will be your everlasting light, And the days of your mourning shall be ended.” (Isa 60:20 NKJV)
Taken by itself, it could be thought to mean the literal sun and moon. But put it into its context:
“19 "The sun shall no longer be your light by day, Nor for brightness shall the moon give light to you; But the LORD will be to you an everlasting light, And your God your glory. 20 Your sun shall no longer go down, Nor shall your moon withdraw itself; For the LORD will be your everlasting light, And the days of your mourning shall be ended.” (Isa 60:19-20 NKJV)
You see? Verse 19 makes clear that this is a situation where the literal sun and moon are no longer giving light. Instead, the Lord will be their everlasting light. That's repeated in verse 20. How could the literal sun be both "no longer be your light" (v19) and "no longer go down" (verse 20)?