This could be multiple reasons
Some parts of the game you cannot manually save. Like some parts of a quest or some areas where things are happening.
For example, you cannot save during the prologue i.e. during the Road to Hogwarts story quest. Creating manual saves will be unlocked once you find yourself for the first time in the chambers of the chosen House in Hogwarts. Similar situations may appear in chosen moments in the further parts of the game.
If your computer's current date and time are set to an older date and time than your latest save's date and time. Your computer's date and time need to be newer than your game's last date and time in order to perform a manual save.
Could be a bug in the game. In that case you need to wait for the developers to fix it.