Enable the safety mode in the CPU object properties under the "Protection" or "F-Configuration" tab.
Before you proceed with the download, verify that your engineering station meets these requirements. Installing V5.6 on an incompatible OS will lead to cryptic error messages. Simatic S7 Distributed Safety V5.6 Download
To achieve full compatibility with running on modern operating systems like Windows 10 (64-bit) , you must install S7 Distributed Safety V5.4 SP5 (with Update 2) paired with the corresponding S7 F ConfigurationPack . Enable the safety mode in the CPU object