What's wrong with my provided admin password for Oracle Apex instance administrator account?
Nothing is wrong with that password. But just to be sure, check the variable if it really contains the password you entered (print :PASSWORD):
Thank you for installing Oracle Application Express 20.1.0.00.13

Oracle Application Express is installed in the APEX_200100 schema.

The structure of the link to the Application Express administration services is as follows:
http://host:port/pls/apex/apex_admin (Oracle HTTP Server with mod_plsql)
http://host:port/apex/apex_admin     (Oracle XML DB HTTP listener with the embedded PL/SQL gateway)
http://host:port/apex/apex_admin     (Oracle REST Data Services)

The structure of the link to the Application Express development interface is as follows:
http://host:port/pls/apex (Oracle HTTP Server with mod_plsql)
http://host:port/apex     (Oracle XML DB HTTP listener with the embedded PL/SQL gateway)
http://host:port/apex     (Oracle REST Data Services)

timing for: Phase 3 (Switch)
Elapsed: 00:00:06.97
timing for: Complete Installation
Elapsed: 00:05:01.54

PL/SQL procedure successfully completed.





1 row selected.

...null1.sql

Then:
SYS&gt; @apxchpwd.sql
...set_appun.sql
================================================================================
This script can be used to change the password of an Application Express
instance administrator. If the user does not yet exist, a user record will be
created.
================================================================================
Enter the administrator's username [ADMIN] apexadmin
User &quot;apexadmin&quot; does not yet exist and will be created.
Enter apexadmin's email [apexadmin] a@b.c
Enter apexadmin's password []
Created instance administrator APEXADMIN.

SYS&gt; print :PASSWORD

PASSWORD
--------------------------------------------------------------------------------
H29dN8%Ih^^IV$Y3$j4Mx

SYS&gt;