Instructions copied from Roaster-Dude https://www.reddit.com/r/Proxmox/comments/tgojp1/removing_proxmox_subscription_notice/?share_id=S5V91w5M7jBQ7jAn2bnwn&utm_content=2&utm_medium=android_app&utm_name=androidcss&utm_source=share&utm_term=14
Similar instruction are available also in https://dannyda.com/2020/05/17/how-to-remove-you-do-not-have-a-valid-subscription-for-this-server-from-proxmox-virtual-environment-6-1-2-proxmox-ve-6-1-2-pve-6-1-2/
Login to proxmox server.
Change folder
cd /usr/share/javascript/proxmox-widget-toolkit
Backup file
cp proxmoxlib.js proxmoxlib.js.bak
Edit file
nano proxmoxlib.js
Change .data.status.toLowerCase() !== 'active')
to .data.status.toLowerCase() == 'active')
Save file
Restart Proxmox e.g.
systemctl restart pveproxy.service