The most likely cause of this is a time model that contains settings that prevent booking. These include, above all, break rules that require intervention when booking. All settings that prevent times from being SAVED when recorded manually in Personio and to which employees have to react by changing times are unsuitable for recording at the terminal. It is not possible to react interactively there and the Personio interface rejects the booking accordingly.
The correct way to deal with this is to configure the time model so that I can record “whatever” times at the terminal and then use automation and workflows within Personio to react to time recordings that violate the desired rules.