Quick answer
Tomorrow is not yours to promise. The verse forbids the boast rather than the planning, and gives the reason in one clause: you do not know.
Read both lines
The prohibition is narrow and worth keeping narrow. It is aimed at boasting, the confident announcement of what will happen, not at preparation, which this book recommends constantly and never apologizes for. The distinction is between arranging for tomorrow and speaking as though you own it.
The second line supplies a reason rather than a threat, and the phrasing is almost gentle: you do not know what a day may bring forth, the verb used of giving birth, so the day is carrying something you have not seen. James lands in the same place and sharpens it, telling merchants who plan a year of trade in a named city that they do not know what will happen tomorrow, and that they should say instead "If the Lord wills, we shall live and do this or that" (4:13–15). He then adds the verdict the proverb leaves out: "But now you boast in your arrogance. All such boasting is evil" (4:16).
Related: James 4:13–16 · Proverbs 16:9 · Proverbs 19:21 · Proverbs 20:24
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.