This has happened a number of times with clients of mine. They do not have iconv integrated with PHP (although it is on their file system). I install phpBB, go into the ACP and get the message shown for example in this topic:

https://www.phpbb.com/community/viewtopic.php?f=556&t=2423316

You cannot do anything inside the ACP until this error is resolved. I've changed the function returning the error to get around it temporarily and access the ACP. But I think that phpBB should

• Identify iconv() module as a system requirement on its requirements page: https://www.phpbb.com/support/docs/en/3.2/ug/
• The installer should not allow installation if it doesn't detect the PHP module enabled