Is there an error number shown along with the text?
Is the computer date and time correct (month. day, year, time, time zone)?
Open Defender and tell us what version the definitions are currently shown as on the update tab, the creation date, and the last updated date/time information.
Is this a new installation? If not, when did this problem begin?
Does Windows Update work?
"open it" meaning -- open Defender or open Action Center?
Troubleshooting General Errors while Updating
-steve