ORA-12557: TNS:protocol adapter not loadable

Error:

C:\TEST>sqlplus sys as sysdba
SQL*Plus: Release 12.1.0.2.0 Production on Sat Jul 4 10:57:50 2020
Copyright (c) 1982, 2014, Oracle. All rights reserved.

Enter password:
ERROR:
ORA-12557: TNS:protocol adapter not loadable


Cause
In windows environment, if you install first oracle database software then after you use Oracle Client Software, then Path variable of windows is updated.

Solution
You need to change the sequence of path variable, first you need to come with Database software path then client software path in PATH environment variable.

1. Open the Environmental PATH setting in windows.

2. Check the Environment PATH variable value, as shown below.
Client path is in first position and DB path is in Second position.

3. Edit the PATH Variable and move the DBHOME Path first and Client path on second position.
Note: I am using latest window version, otherwise you manually copy paste the path variable in notepad and update and copy paste back in Environment variable.

4. Save and Press OK and OK and return from Environment windows.

5. For test changes open the new command prompt and test.
Hopefully it will fixed the issue.