Searching for ""outsystems"."DBO".ossys_user_t2 with key 0 was not found"

Please let me know the solution for this error while sending email.

I have not renamed the db Name... Which could have the possibility for this error.
You're passing a NullIdentifier somewhere in your flow. 
Check the parameters that you are passing in the FROM and TO of the Send Email action.
Make sure you aren't doing something like GetUser(Id).USER.Email and the Id is null.
Don't use the Session.UserId variable in your Processes or Logic-actions.
I'm getting this error.... " [AM3C79069].DBO.[OSUSR_4Z4_EMPLOYEEE_DETAILS1] with key 0 was not found " What should i do to rectify this? Please help me to sort it...

You're executing action GetEmployee_Details with an empty (null) Id. This will throw the exception above.