Prerequisite:
You need to know how to SSH.
and/or
Need to know how to work in the Linux terminal.
Basic Linux knowledge.

To remove the message, SSH to your server, or use the terminal nyt starting the terminal on your PVE node.

The easy way, is to use the following command:
sed -Ezi.bak "s/(Ext.Msg.show({\s+title: gettext('No valid sub)/void({ \/\/\1/g" /usr/share/javascript/proxmox-widget-toolkit/proxmoxlib.js && systemctl restart pveproxy.service

And verify it with:
grep -n -B 1 'No valid sub' proxmoxlib.js

If you want to do it manually, this is the steps.

1. Change directory:
   cd /usr/share/javascript/proxmox-widget-toolkit

1. Make a backup:
   cp proxmoxlib.js proxmoxlib.js.bak

1. Edit the file:
   nano proxmoxlib.js

1. Find the code snippet (use CTRL + W to search with nano):
   Ext.Msg.show({
title: gettext('No valid subscription'),

1. Replace the code in step 4 with:
   void({ //Ext.Msg.show({
title: gettext('No valid subscription'),

1. Restart the Proxmox Web Service (you might need to refresh your browser cache as well):
   systemctl restart pveproxy.service

In case you ned to revert the changes, you have 3 options.

1. Manually edit the file from the manual steps, just swap the code snippinsg step 4 and 5.
2. Restore from the backup file in step 2.
3. Reinstall the proxmox-widget-toolkit package the repository with the following command:
   apt-get install –reinstall proxmox-widget-toolkit