Installed pgadmin4-apache2 package post-installation script subprocess returned error exit status 1

I’m trying to install pgAdmin4 desktop mode, Facing this Issue. It show pgAdmin4 and pgadmin4-apache2 installed. but while opening the pgAdmin4 application it display an error on popup

Failed to launch the application server, server thread exiting.

Although the command for installing pgAdmin4 and pgadmin-apache2

sudo apt-get install pgadmin4 pgadmin4-apache2 

Show the following error.

pgadmin4 is already the newest version (4.8-3.pgdg18.04+1).
pgadmin4-apache2 is already the newest version (4.8-3.pgdg18.04+1).
The following packages were automatically installed and are no longer required:
  apache2-data apache2-utils libprotobuf-c1 php7.2-cli php7.2-json php7.2-opcache php7.2-readline
Use 'sudo apt autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 14 not upgraded.
1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] y
Setting up pgadmin4-apache2 (4.8-3.pgdg18.04+1) ...
WARNING: Failed to set ACL on the directory containing the configuration database: [Errno 1] Operation not permitted: '/var/www/.pgadmin'
Traceback (most recent call last):
  File "setup.py", line 413, in <module>
    setup_db()
  File "setup.py", line 345, in setup_db
    create_app_data_directory(config)
  File "/usr/share/pgadmin4/web/pgadmin/setup/data_directory.py", line 42, in create_app_data_directory
    os.chmod(config.SESSION_DB_PATH, 0o700)
PermissionError: [Errno 1] Operation not permitted: '/var/www/.pgadmin/sessions'
dpkg: error processing package pgadmin4-apache2 (--configure):
 installed pgadmin4-apache2 package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
 pgadmin4-apache2E: Sub-process /usr/bin/dpkg returned an error code (1)