Is it legal to use SPY24?

It is perfectly legal to use SPY24:

Like a parental solution to monitor your minor children.
Like a tracking solution used to monitor the devices belonging to the company. To avoid legal problems, be sure to inform the staff and to get their permission for the use of the application on devices that the company owns.
We are not, however, able to provide comprehensive legal advice on the specificity of some local laws.

In most cases, it is against the law to install monitoring software on a cell phone or any other device without having received express permission. You are required to inform users of the device that they are under surveillance. Any breach of this obligation is your own responsibility and the installation of the application is at your own risk.