Maria Horton was given the card by the charity Family Fund to buy an iPad to help her son Sam, four, with his school work.
But when she visited a Comet story in Plymouth, staff refused to accept it.
Mrs Horton, 32, from Plymouth told Radio Devon: "I said to the staff, 'You already have the money, can't you accept it?' and they said, 'We can't do anything for you.'
"I spoke to the manager and he said the same thing, that I would have to go away without the goods.
"I'm not the only one who has lost out. They have the money but they are not willing to provide the goods to the children who need them."
Sam has cerebral palsy after his brain was starved of oxygen during his birth. He was born by emergency caesarean section seven weeks prematurely after Maria suffered from hypertension.
At five months, doctors detected abnormalities with his physical development and he was diagnosed with cerebral palsy at 15 months.
Comet has appointed Deloitte as administrators and 6,611 jobs are at risk.
Deloitte has said that that customer orders will be fulfilled 'wherever possible', but the use of gift cards and vouchers was suspended on Saturday November 3.
Family Fund was unavailable for comment.