Lean 4

rw: did not find the pattern (Lean 4)

rw looked for the left-hand side of your equation in the goal and it is not there — at least not in exactly that form.

What it looks like

Main.lean:7:6: error: Tactic `rewrite` failed: Did not find an occurrence of the pattern
  b + a
in the target expression
  a + b = c

a b c : ℕ
h : b + a = c
⊢ a + b = c

What does "rw: did not find the pattern (Lean 4)" mean?

rw needs the left-hand side of the equation to appear in the goal in essentially the same form; it only unfolds reducible definitions while looking. Lean 4.23 and later print Tactic `rewrite` failed: Did not find an occurrence of the pattern … in the target expression …, followed by the goal; Lean 4.22 and earlier print tactic 'rewrite' failed, did not find instance of the pattern in the target expression.

What causes "rw: did not find the pattern (Lean 4)"?

The goal has a different form of the term (b + a instead of a + b, or different parentheses).
The term is hidden inside a definition that rw does not unfold.
The occurrence is under a binder (inside fun, ∀ or ∃) and mentions the bound variable — rw cannot rewrite there.
The equation is used in the wrong direction, or the term is in a hypothesis rather than the goal.

How do I confirm the cause?

Compare the pattern with the goal: look for swapped arguments, extra parentheses or a coercion arrow ↑.
Hover over both terms — implicit arguments that differ are the usual hidden cause.
Check whether you meant the goal or a hypothesis (rw … at h).

How do I fix "rw: did not find the pattern (Lean 4)"?

Rewrite right-to-left with rw [← h] if the goal contains the right-hand side.
Rewrite in a hypothesis with rw [h] at h2.
Unfold the definition first (unfold f or simp only [f]), or restate the goal with show.
Use simp only [h], which can also rewrite under binders.

How do I stop it happening again?

Put both sides in a normal form first — simp, or ring_nf in Mathlib — before rewriting.
Keep lemma statements in the same form as the goals you use them on.

Got a different error? Paste it into the Explain This Error tool → Identifies the family and the fix — runs locally, nothing uploaded.