You might want to update the question wth more information about the error message and other details. 

Now i couldn't login to exercise system, getting the below mentoined error

Cannot save Data-Admin-Operator-ID instance to database.com.pega.pegarules.pub.database.DatabaseException: Database-General Problem writing an instance to the database 0 53100 ERROR: could not extend file "base/16384/3509449": No space left on device Hint: Check free disk space. DatabaseException caused by prior exception: org.postgresql.util.PSQLException: ERROR: could not extend file "base/16384/3509449": No space left on device Hint: Check free disk space. | SQL Code: 0 | SQL State: 53100 From: (H75A6A7606CF71008D08D0045F3C500C5:124.123.161.106) SQL: UPDATE pegadata.pr_operators SET pxcreatedatetime = ? , pxcreateoperator = ? , pxcreateopname = ? , pxcreatesystemid = ? , pxinsname = ? , pxobjclass = ? , pxsavedatetime = ? , pxupdatedatetime = ? , pxupdateoperator = ? , pxupdateopname = ? , pxupdatesystemid = ? , pyaccessgroup = ? , pyallowrulecheckout = ? , pycalendar = ? , pydefaulttimezone = ? , pyemailaddress = ? , pyextension = ? , pyimagefilename = ? , pylabel = ? , pylastsignon = ? , pylicensetype = ? , pyopavailable = ? , pyorganization = ? , pyorgdivision = ? , pyorgunit = ? , pyposition = ? , pyreportto = ? , pyrulesetname = ? , pyuseridentifier = ? , pyusername = ? , pyworkgroup = ? , pxCommitDateTime = CURRENT_TIMESTAMP , pzPVStream = ? WHERE pzinskey = ? AND pzTenantId = ? SQL Inserts: <2016-06-22 21:04:52.268> <Admin@TGB> <Administrator> <pega> <SA@TGB> <Data-Admin-Operator-ID> <2016-10-23 07:51:53.509> <2016-07-19 15:30:47.68> <SystemArchitect@TGB> <System Architect> <pega> <HRAppsSAE:Administrators> <true> <USDefault> <America/New_York> <SA@TGB> <<null>> <<null>> <System Architect> <2016-10-23 07:51:53.42> <NAMED> <true> <TGB> <Div> <Unit> <<null>> <Manager@TGB> <TGB> <SA@TGB> <System Architect> <default@TGB> <[B@3b7e6c97> <DATA-ADMIN-OPERATOR-ID SA@TGB> <645a8a0c6b067567e576255b5f19cc52> Caused by SQL Problems. Problem #1, SQLState 53100, Error code 0: org.postgresql.util.PSQLException: ERROR: could not extend file "base/16384/3509449": No space left on device Hint: Check free disk space.

You are running out of the Disk space to perform a db save. Try to cleanup the device and make some space. 

I think you should have allocated more space while creation of the excercise system.

Ok, i could login  now but when i create a property i'm getting 

i am using online exercise system where can i free disk space here. 

You are still in need of free disk space. You should allocate more space in the VM settings.

Pega Exercise system takes 5GB space by default. If you want to have more space you need to allocate it at the time of exercise system creation. (This can be done in the advanced settings of the VM instance creation)

Download Personal edition which uses your system DB space