All the spaces have already been allocated for the session, which runs from 9am to 3.30pm, so the club has said it will arrange another session as soon as possible.
Longmead has seen more than 1,000 young people attend such events, which are run by Football Fightback, a campaign to eradicate undetected heart conditions.
It was started by the Angels four years ago after the sudden death of two young footballers from previously undetected heart defects.
The club and the community have raised more than £35,000 to help fund the tests.
In 2015 Junior Dian collapsed on the pitch and died during a game at Whyteleafe aged 24. Seven years before, Angels’ Under-18 player Jack Maddams, 17, died in his sleep.
The testing, which usually costs hundreds of pounds, is facilitated by Cardiac Risk in the Young.
It is the same procedure as those which have been carried out at Manchester City, Tottenham Hotspur and the England rugby team.