It is the La Liga has restricted Barcelona from registering these players. This season without a good financial standing, the club has spent more than any other club in Spain in the transfer market. Currently they are unable to register those players because the La Liga officials discovered that they don't have the funds they claim they possess. Part of the money they claimed they received from investors were actually paid by the club using another company. Now they have to source for other means to raise funds that would enable them register those players.
Old story. It is why they let Messi leaves their club 2 seasons ago. Now, they are in a same story but their finance are at risk again. The leave of Messi can not help them and their director board is taking higher risk now.
The restriction of La Liga forces Barcelona to find exit clubs for many players. To reduce their salary burden and to register new players they signed in this summer.
Currently they have to sell some of their players to still comply with financial fair play standards. It seems that they have to sell some players immediately. Many media outlets gave news that they were ready to expel De jong, Memphis Depay, Braithwaite, Neto, and Mingueza.
If the name doesn't go away of course they can't register new players. Surely with this matter there will be new news in the transfer market before it closes.
Barcelona are in a big gamble again. They will come back to be a giant and improve their finance or they will go more closely to a verge of collapse and bankruptcy.
I don't know what are best solutions for them but Laporta and his colleagues are taking very high risk. Furthermore, they are losing faith from their players. The story between Barcelona and De Jong, and 2 seasons ago, with Messi, are very sad and bad. I don't think they should not be a host of such bad drama.
No matter what they told in press conference or on media, they actually gave De Jong very unfair and bad treatment.