If you have been using SharePoint Designer to create and edit custom forms in SharePoint Online, you can no longer do it since April 25th, 2020.

This change has not been communicated earlier.

If you need to create or edit custom form in SharePoint Online, you need to switch to PowerApps.

Users still using the Classic SharePoint Online experience will have to temporarily switch to the Modern Experience to be able to start using PowerApps; then they can switch back to the classic (while the recommendation is to use the Modern Experience).