There are at least 3 possibilities causing “ORA-12545: Network Transport: Unable to resolve connect host name” in Visual Studio.
Your host name is wrong in tnsnames.ora file.
(ADDRESS = (PROTOCOL = TCP)(HOST = servername)(PORT = 1521))
(SERVER = DEDICATED)
(SERVICE_NAME = orcl)
Put the name and SERVICE_NMAE the same for example orcl.
Choose EZ Connect as the Connection Type. Once you can connect then switch back to TNS