In all relatively recent site collections you will find a Deny permission on Add and customize pages. This permission rule prevents changes to site pages as a security measure. Unfortunately, it also means that you won't be able to customize your list forms using Ultimate Forms. You will be notified through a warning banner within the app if the permission is present within your site collection.

1. If you haven't done so, install SharePoint Online Management Shell
2. Run the following commands, replace (for example, if your site is https://example.sharepoint.com, your tenant name is "example", so the full tenant URL would be https://example-admin.sharepoint.com) with your tenant name and with the site URL, such as https://example.sharepoint.com/sites/sample. Note that your site URL and the tenant URL are not the same.
   1. Connect-SPOService -Url "https://[your tenant]-admin.sharepoint.com"
   2. Set-SPOSite -Identity -DenyAddAndCustomizePages $false

NOTE: this setting will remain for 24 hours, after which SharePoint will automatically reset them back to Deny. It is not possible to make this setting remain for longer than 24 hours. We suggest you ensure you are using version 1.4.0.0 and modern forms, which do not require this permission.

NOTE: When using versions prior to 1.4.0.0, make sure Site Pages feature is activated in any site where you plan to use Modern Forms. In most cases, it will already be activated. If you are having issues publishing forms, go to Site Settings -> Site Features and ensure Site Pages feature is activated.