So, you have installed Proxmox and when you login you see:

PVE provides free version. The Paid versions offer support and are highly recommended if you are using Proxmox for production. To see the paid version go here:https://www.proxmox.com/en/proxmox-ve/pricing

So, to remove the popup run the following:

Backup the file /usr/share/javascript/proxmox-widget-toolkit/proxmoxlib.js

cp /usr/share/javascript/proxmox-widget-toolkit/proxmoxlib.js /usr/share/javascript/proxmox-widget-toolkit/proxmoxlib.js.backup

Now lets edit the file.

nano proxmoxlib.js

Look around line 512 for the following string. You can use crtl-w if using nano to search.

if (res === null || res === undefined || !res || res
                        .data.status.toLowerCase() !== 'active') {
                        Ext.Msg.show({
                            title: gettext('No valid subscription'),

Replace the following string with the string below:

if (res === null || res === undefined || !res || res
                        .data.status.toLowerCase() !== 'active') {

Replace with this:

if (false) {

Here is what it should look like:

 if (false) {
                        Ext.Msg.show({
                            title: gettext('No valid subscription'),

Restart Proxmox service

systemctl restart pveproxy.service

Log out of Proxmox, clear the browser cache and restart the browser then login again. The popup should be gone.