This one worked for me! Drupal 8 Unable to find install new module option after a latest installation of Drupal 8.* (Not sure if it works for D9**)
step 1 : Check if the Update manager module is enabled, if its not unable then enable it and click Install. Go to admin panel and see if you can view Install new module button If its still not visible then go to step 2.
Step 2 : If you still unable to see button Install new module then open url for e.g. http://yourprojectname.com/update.php or http://localhost/drupalsite/update.php
Use update.php utility to update your database whenever a new release of Drupal or a module is installed. Please read all the instructions before pressing Continue button.
Once Continue button is pressed, it will take some time to perform update operation, On complete it will show a message "No pending updates."
Step 3: Go to admin panel and see if can view Install new module button
https://github.com/SatheMangesh/Drupal8_9/blob/master/qa/Drupal8notes.txt