To configure the vsftpd server to allow only uploads and not downloads, you can set the download_enable directive in the server's configuration file to NO.
The download_enable directive controls whether clients are allowed to download files from the server. When set to NO, clients will not be able to download files from the server.
To set the download_enable directive to NO, open the server's configuration file, which is typically located at /etc/vsftpd.conf, and add the following line:
download_enable=NO
Save the configuration file and restart the vsftpd server for the changes to take effect.
You can find more information about the download_enable directive and other configuration options for the vsftpd server in the vsftpd man page by running man vsftpd in a terminal.