Oracle 19c cannot create master key
So what instructions did you follow?

Below is an example with your commands, where I did not use sqlnet.ora at all, because it is optional in 19c:

SQL&gt; !mkdir /u01/app/oracle/admin/MIN19_O71/wallet

SQL&gt; alter system set wallet_root='/u01/app/oracle/admin/MIN19_O71/wallet' scope=spfile;

System altered.

SQL&gt; startup force
ORACLE instance started.

Total System Global Area 1073737800 bytes
Fixed Size                  8904776 bytes
Variable Size             297795584 bytes
Database Buffers          759169024 bytes
Redo Buffers                7868416 bytes
Database mounted.
Database opened.
SQL&gt; alter system set tde_configuration="keystore_configuration=file" scope=both;

System altered.

SQL&gt; administer key management create keystore identified by "mypassword";

keystore altered.

SQL&gt; administer key management set keystore OPEN identified by "mypassword";

keystore altered.

SQL&gt; !ls -l /u01/app/oracle/admin/MIN19_O71/wallet
total 0
drwxr-x---. 2 oracle oinstall 25 Dec 30 16:01 tde

SQL&gt; !ls -l /u01/app/oracle/admin/MIN19_O71/wallet/tde
total 4
-rw-------. 1 oracle oinstall 2555 Dec 30 16:01 ewallet.p12

SQL&gt; administer key management set key using tag 'master key' identified by "mypassword" with backup using 'masterbackup';

keystore altered.

SQL&gt; !ls -l /u01/app/oracle/admin/MIN19_O71/wallet/tde
total 12
-rw-------. 1 oracle oinstall 2555 Dec 30 16:02 ewallet_2019123015024590_masterbackup.p12
-rw-------. 1 oracle oinstall 4171 Dec 30 16:02 ewallet.p12

SQL&gt;