What are the thick bars around the outside?

Bearer bars. Interleaved 2 of 5 can be misread as a shorter valid code if a scanner clips the edge, and the frame makes a partial read impossible.

Why does it need an even number of digits?

The format encodes two digits at once, one in the bars and one in the spaces between them. That interleaving is where its density comes from, and it cannot work on an odd count.

Why will my printed barcode not scan?

Usually size or the margin. Print from the SVG at its intended size, keep the clear space either side, and use black on white rather than a colour.

ITF-14 Barcode Generator

The barcode on the outer case, not the item.

Digits

Thirteen digits, or fourteen including the check digit.

Print the value belowYes

Bar height60px

The barcode on the outer case, not the item.

How to use it

Enter thirteen digits. The fourteenth is the check digit and is worked out for you. Paste fourteen and it is verified instead.
Check the line under the code. It says whether the check digit was calculated or verified. A mismatch is refused rather than drawn.
Download the SVG. Print at the size the carton needs. The bearer bars around the symbol are part of the image.

When you would use this

ITF-14 is the barcode on the outer case rather than the item inside it, which is why you see it on cardboard in a warehouse and never on a shelf. Fourteen digits, the last calculated from the other thirteen in the same way as an EAN. The encoding is interleaved: each pair of digits is drawn as one group with the first digit in the bars and the second in the spaces between them, which halves the width and is why the digit count has to be even. Bearer bars frame the symbol, and they are structural rather than decorative: a partial scan across a clipped edge can otherwise read as a shorter code that happens to be valid, and on printed cardboard that is a real risk.

Questions

What are the thick bars around the outside?: Bearer bars. Interleaved 2 of 5 can be misread as a shorter valid code if a scanner clips the edge, and the frame makes a partial read impossible.
Why does it need an even number of digits?: The format encodes two digits at once, one in the bars and one in the spaces between them. That interleaving is where its density comes from, and it cannot work on an odd count.
Why will my printed barcode not scan?: Usually size or the margin. Print from the SVG at its intended size, keep the clear space either side, and use black on white rather than a colour.