The following error appeared when you are trying to conenct to database using toad applicaion: –

 

Make sure of the following :-

• Database is up and running
• Listener is up and database is registered using.

lsnrctl status

if the above steps is done and still facing the same issue then do the following :-

• Right Click on my computer and choose properties.
• Advance system settings.
• Environment Variable.

check the below entry is exsits, if not add it by press on new and follow the same steps by adding TNS_ADMIN and the location of the tnsnames.ora, sqlnet.ora into the 2nd box like the picture below.

 

Thanks

Osama