Which OpenCart releases this runs on

Gift Cards installs on OpenCart 4.0.2.0 and the rest of the 4.0.2.x line. It refuses to install on 4.1.0.0 and above, and that is deliberate rather than a gap somebody has not got to yet.

Why there is a ceiling at all

OpenCart removed the gift-voucher feature outright in 4.1.0.0. Not renamed and not deprecated — the model that resolves a code, the apply-code box at checkout and all four voucher tables are simply not in the release. A 4.1 store cannot sell a gift card, so there is nothing for this extension to add an expiry date to, no balance for a holder to look up and no unspent money to report.

Most extensions in this range warn about an untested release and carry on, because a store on a newer OpenCart usually still gets most of what it paid for. That is not true here: on 4.1 you would get two empty tables and three screens with nothing to say. Refusing tells you that on the day you install it rather than on the day you go looking for a card.

What it actually checks

The check is for OpenCart's own voucher storage, not for a version number. If a future OpenCart brings gift vouchers back, this extension will install on it without us shipping anything — and if a release you are on has vouchers but is newer than anything we have tested, it installs and says so in its log rather than refusing.

Your store	What happens
Older than 4.0.2.0	Refused. Nothing has been tested below it.
4.0.2.0 to 4.0.2.3	Supported: a full install-to-uninstall pass has been run on it.
A later release that still has vouchers	Installs, and writes a line to the extension's log saying the release is untested. No screen says so.
4.1.0.0 or later, with no vouchers	Refused, with the reason below.

What a refused store is told

this store has no gift vouchers to extend: OpenCart removed the voucher subsystem in 4.1.0.0, and Gift Cards is a layer over it rather than a replacement for it. It runs on the 4.0.2.0 line, where vouchers are still part of OpenCart.

The line is written to the Kyvero diagnostic log, which you will find under System > Maintenance > Error Logs in the kyvero.log tab. Nothing else on the store is changed: no tables are created, no settings are written and your vouchers — if the release has any — are untouched.