Here is what Apple Support says about this issue.
https://support.apple.com/en-us/HT204937
macOS
Do you have any anti-virus software installed on your Mac?
Parental control set?
Is system time set correctly?
For more info:
https://support.apple.com/en-us/HT204098
Section 5: https://support.apple.com/en-us/HT201400