When I implement database mirroring in SQL server it shows an error
I would suggest you to verify the service accounts you are using:


SQL Server service account requires CONNECT permissions to partners’
endpoints
Best option is to use domain accounts for all partners
If using Local Service or Local System, must use certificate
authentication.



  Ensure that the start date of the certificate is the current date or a
  day in the past relative to all servers. Due to time zone differences,
  if the current date on the server is earlier than the start date of
  the certificate, endpoint authentication will fail. Set the START_DATE
  option of the certificate to the day prior to when you are creating it
  to ensure it will work in all time zones.



If using Network Service, must use the computer account
(domain\Computer$) Windows permissions are irrelevant. Do NOT add to 
local admins group
Check the SQL log for errors


Also check for  SQL Server service account in case it  do not exist in logins on both Principal &amp; Mirror

You can fix for above case by adding the service account in logins on both the servers and grant the connect permission as below;

GRANT CONNECT ON ENDPOINT::Mirroring TO [domain\loginname]


For further troubleshooting in case above does not help read here