Prince Harry and Meghan's charitable Archewell Foundation has been listed as "delinquent" by US authorities, after a cheque for registration failed to arrive.
A spokesman for the couple says all the charity documents were filed on time, but the required cheque appears not to have been received.
It has been re-sent so the issue will be "quickly resolved", says Archewell.
Until then Archewell is not allowed to operate as a charity.
The letter from the State of California Department of Justice, sent on 3 May, gives a "delinquency notice" to the Archewell Foundation.
It warns that the process for being on the "registry of charities and fundraisers" has not been completed and as such Archewell cannot carry out charitable activities, such as "soliciting or disbursing charitable funds".
The charity, which gave grants of $1.2m (£956,000) last year, says that a cheque sent alongside registration information was not received by the California authorities, triggering the "delinquency" notice.
A spokesman for Prince Harry and Meghan said that once Archewell had been notified, a "new cheque has been mailed".
This registration problem was not about tax filings, which a spokesman said had been submitted in full and paid on time.
Tax filings for 2022, published last year, showed Archewell had holdings of $11m (£9m) and made donations including $100,000 (£80,000) for the Halo Trust, a mine-clearing charity once supported by Princess Diana.
Other funded projects included $200,000 (£160,000) for a "gender justice" project in Washington, $125,000 (£100,000) for a civil-rights charity and $100,000 (£80,000) for a project promoting responsible use of technology.
Archewell was founded when Prince Harry and Meghan moved to the US and stepped down from being working royals.
Earlier this year there was an overhaul of the couple's operation, with a new sussex.com website, under the title of the Office of Prince Harry and Meghan.
The couple have just completed a visit to Nigeria, following celebrations in the UK to mark the 10th anniversary of the Invictus Games.