why do the error messages then appear?
These are just notices from the jail creation script. They are ok.
When you use a jailed cronjob, then all apps that you want to execute have to be in the jail. The php binary is not in the jail, so executing a php file with /usr/bin/php /path/to/my/file.php will not work.
There are several possible solutions:
a) When you want to use a jail, then use a command that is available inside the jail like wget to execute the php. The php script has to be put in the web dir so that wget can fetch it with a http url.
b) install php into the jail. Thats a bit complicated as php jhas a lot of dependencies. You might want to consult the jailkit doczúmentation if you like to do this.
c) Do not enable jails for thsi website. But thats not as secure as all system commands and files that can be accessed by a normal not jailed shell user are accessible by the cronjob.