Why does VLOOKUP return #N/A when the value exists?

Almost always because of an invisible difference: an extra space, a number stored as text on one side only, or a lookup range that slid when copied because it lacked $.

Should I use IFERROR or IFNA?

IFNA around a lookup: it only hides "not found". IFERROR hides every error, including those that point to a broken formula.

The #N/A error

It is the lookup error: VLOOKUP, XLOOKUP, MATCH or INDEX did not find the value requested. Nine times out of ten, the value is there, just written slightly differently.

What it means

#N/A means "not available". A lookup function went through the range without finding the value, and says so rather than returning a wrong result.

It is the most useful of the errors: it points to a real gap in the data. Hiding it without understanding it throws that information away.

Where it comes from

Extra spaces: "Smith" and "Smith " (with a trailing space) are not the same value. Software exports are full of them, non-breaking spaces included.
A number stored as text on one side: code 1024 is a number in one file and text in the other (small green triangle). For Excel, they differ.
The lookup range slid: the formula was copied down without $, and B2:E50 became B40:E88. The first values are no longer in the range.
The value really is missing: a new customer, a mistyped code.
An approximate match on an unsorted list: VLOOKUP without a fourth argument looks for an approximate value and can return #N/A (or worse, a wrong result).
The NA() function, written on purpose, often so that a chart skips a point.

Fixing it

End VLOOKUP with ,FALSE for an exact lookup: =VLOOKUP(A2,Customers!$A:$C,3,FALSE).
Clean both compared columns: =TRIM(A2), or Clean text on the whole file.
Give both sides the same type: Convert text to numbers, or =VALUE(A2) in the formula.
Lock the lookup range with $ before copying: $B$2:$E$50.
Check that the value exists: =COUNTIF(Customers!A:A,A2) returns 0 if it is missing.

Hiding it, when it is expected

When absence is normal (a product with no discount, a customer with no order), =IFNA(VLOOKUP(…),"") shows an empty cell. IFNA only hides #N/A; IFERROR would also hide a typo or a broken range.

With XLOOKUP, the fourth argument says what to return: =XLOOKUP(A2,Customers!A:A,Customers!C:C,"Unknown").

Tools that help

Look up between two files: VLOOKUP between two files, with no formula, which neutralises spaces, case and numbers stored as text, and lists the keys not found.
Explain a formula: flags the approximate match and the sliding range.
Find the right formula: the lookup written for you, locked ranges included.

Frequently asked questions

Why does VLOOKUP return #N/A when the value exists?: Almost always because of an invisible difference: an extra space, a number stored as text on one side only, or a lookup range that slid when copied because it lacked $.
Should I use IFERROR or IFNA?: IFNA around a lookup: it only hides "not found". IFERROR hides every error, including those that point to a broken formula.