It's one of those sentences that almost doesn't make any sense.
26 supposedly U17 players have been deem ineligible after failing an age test. Nigeria, who have been caught once for fielding overage players at youth tournaments, and under suspicion on two other occasions have been forced to leave the players behind.
26 of the 60 in the squad failed the mandatory MRI test. The test is considered to be 99% accurate, and looks at whether the bones in a players arm or leg have fused together. If it's completely fused then the player is an adult, and deemed to be over 18.
A Nigerian official commented on the situation:
It is a disaster. Virtually the first team the coaches have been working on have been disqualified. It’s a serious problem.
Nigeria have won the FIFA U17 World Cup a record five times - most recently in 2013, and 2015.