I met the same issue. You can try the below solutions to fix it.
Solution 1: Retry after Some Time
Solution 2: Restart your Device
Solution 3: Network Issue
Solution 4: Lock/ Unlock SIM Status
Solution 5: Activate Via iTunes
Solution 6: Restore iPhone to Factory Settings before Activation
For more details, check:
How to Fix "iPhone Activation Server Cannot Be Reached"
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