After seeing at client-side this error message:
EN: "TCC cannot suggest a position for this student. Please contact the TCC operator to free up a position."
FR: "TCC ne peut pas proposer un poste pour cet étudiant. S'il vous plait contacter l'operateur du TCC pur libérer un poste."
You can solve this issue by following these steps:
1. Log in to the TCC as admin.
2. On the left-hand side you now see a new option:
"Edit class layouts", you need the "Identify student computers" icon (black).
3. Free up positions by selecting the position you want, and then delete them (red cross), as illustrated in this screenshot:
4. Now it should be possible again to identify new student PCs.