Author uploading file showing http error

[Vitaliy]

This error is typically caused by an absence of access to files dir from a web server (apache). The case may be the wrong path to files dir in config.inc.php:
https://github.com/pkp/ojs/blob/master/config.TEMPLATE.inc.php#L209
Or an absence of permissions to this folder/files:

How should file permissions be set?

There isn’t a straightforward answer because so much depends on your hosting environment. Start with your hosting provider’s documentation and support, or with your system administrator, and with the following instructions: How to set your permissions In general, you want your permissions set such that your webserver can read and write (recursively) to config.inc.php's files_dir, and to ./cache/, and ./public/. Optionally, for added features and reduced security, you can enable write to confi…

I understand your concern but I can’t help you any further.