ORA-12545: Connect failed because target host or object does not exist

It's a very common error while connecting to the Database though any Oracle tool.

Few points to check.

1> Please check if you have the correct DB entry in the tnsnames.ora file.
     > If haven't, create a common TNSNAMES.ORA shared amongst all Oracle Homes
            a) Create c:\Oracle\TNS_ADMIN directory.
            b) Create TNS_ADMIN environment variable:

            c) Place the tnsnames.ora file in c:\Oracle\TNS_ADMIN directory with the 

                correct entry

2> Check if you have the related database connection (dbc) in place
      > If haven't, create a common FND_SECURE  home
            a) Create c:\Oracle\FND_SECURE directory.
            b) Create FND_SECURE environment variable:

            c) Place the related .dbc file in the c:\Oracle\FND_SECURE directory
                     > 11i path: $FND_TOP/secure
                     > R12 path: $INST_TOP/appl/fnd/12.0.0/secure
                     > If you do not have access to Unix box, check with your DBA

3> Please check if you have the correct host entry in your local Host file.
     Host entry must be matching with the host name in the tnsnames.ora file.
      > Host file location: C:\Windows\System32\drivers\etc

4> Most important one :)
     > If you are connecting remotely check if you are connected to the client VPN.