When you book in sessions on a child and then press next you will always get taken to a child's room movement page. If one of the rooms says 'Unsuitable' then either the age a child leaves your nursery or the room configuration is not correct as at this child's maximum age they are too old for your oldest room.
When booking in sessions
Here you can see that when the child gets to 2014 there is no room suitable for them.
Check your leaving age of children.
Click on 'Settings' and then 'Configuration'.
1) Choose the date in the month children leave the nursery.
2) Choose the age that they leave.
3) Click Save.
Check your room configuration.
Click on 'Settings' and then on 'Rooms'.
1) Check the maximum ages in your rooms and in your last room.
2) To edit press the edit pencil.
3) Change the Maximum Age to what you want it to be.
4) Click Save.
Once you have set this up children should not come up in an unsuitable room as they will fit into one of your other rooms at your nursery.