You might try installing toad into a directory that does not contain a parenthesis - as there were Oracle client issues with parenthesis in name. If you search the toad support web site you’ll find references to this known Oracle client issue. I copied one such support doc below - note solution #5:
Toad for Oracle Knowledge Article 35757
Title Error attempting to connect to database via Toad "ORA-12154 TNS: Could not resolve the connect id
ORA-12154 TNS: Could not resolve the connect identifier specified error upon attempting to connect to database via Toad.
Check for usual issues with 64bit.
If not, then may be an issue with tns and sqlnet setup.
If Oracle client installed is a 64-bit client.Toad will not show this client under Installed Clients of the Login window. Oracle client needs to be uninstalled and a 32-bit client needs to be installed on machine. Attached is documentation on how to completely remove the existing client. We cannot guarantee the contents of this documentation, and it is for users information only.
To download a free version of Oracle client, refer to www.oracle.com/…/index.html.
If the 32-bit client was recently upgraded. Perform the following:
Right-click on MY COMPUTER and select Properties
Select Advanced Tab and Environment Variables button
Under System Variables, if an ORA_HOME entry exits ensure the BIN folder of the Oracle client exists for this entry
Hit Edit button to modify or add this path
Right-Click My Computer | Properties | Advanced tab | Environment Variables button | under System Variables section | click New button.
Type: TNS_ADMIN for Variable Name.
Click OK, OK, and OK
Start Toad and re-test issue
Check the sqlnet.ora file. If there is “NAMES.DEFAULT_DOMAIN = <>”, please make sure the domain specified is included in you database entries in the tnsnames.ora file. Or you can remove this line from the sqlnet.ora file so there is not domain requirement.
NAMES.DEFAULT_DOMAIN = PROD.QUEST
(ADDRESS = (PROTOCOL = TCP)(HOST = TEST206115)(PORT = 1521))
(SERVER = DEDICATED)
(SERVICE_NAME = test.prod.quest)
If Toad is installed in the Program Files (X86), then copy the Quest Software folder from Program Files (X86) to the Program Files folder and create a short cut to the Toad.exe file from this directory.
Ensure you have a “tnsnames.ora” file and it contains correct and accurate TNS entry for the database you want to connect to. Please consult with your Oracle DBA or IT Department on this regard. They may provide you with a “tnsnames.ora” file you can use.