A player preparing for Forever ran into this. Their subscription had expired and their gold was in the Warband bank. The distinction matters because Forever access is included with a WoW subscription or game time.
This is about using Retail gold to fund game time, not buying tokens inside Forever.
What counts when game time has expired?
Blizzard's token guide says the character-selection purchase combines gold held by characters on one realm, regardless of faction. Mailbox gold also counts. Warband bank gold does not.
If you already have a token bought with gold in a character's inventory, you can redeem it from character selection after game time expires.
The purchase has other conditions. You must have bought and used at least one month of game time with real money since 2017, and that first paid month must have fully elapsed. Your WoW region must also match your Battle.net account's country.
Don't rely on a free day to retrieve the gold
Blizzard explicitly says Customer Support won't provide game time to collect gold. Old advice to ask for a temporary day isn't a reliable solution to this situation.
Before your paid time ends, check where your gold is stored if you plan to renew with a token. Keeping the required amount on characters on one realm avoids this particular Warband bank restriction. It doesn't remove the other eligibility checks.
If you're also waiting for a physical box, the Collector's Edition access guide covers that separate question.