Therefore, and also because it is best practice to know and determine the wallet location deliberately (since it is a pretty important file), please consider to always set the ENCRYPTION_WALLET_LOCATION in the sqlnet.ora file.
Comment: If you have specified an ENCRYPTION_WALLET_LOCATION in your sqlnet.ora file you should not have the problem writing to any default location, in that case check for the proper format and indentation of your sqlnet.ora file.
In each case make sure the directory exists on the filesystem and is accessible by the oracle software owner.
2. Create the wallet file and set the encryption key :
SQL> ! ls -l admin/WALLET
-rw------- 1 oracle dba 1309 Oct 13 10:31 ewallet.p12