1. Verify three forms. If you understand how SharePoint works, you will know immediately that this should be the issue on SharePoint three form issue (NewForm.aspx, DidForm.aspx, and EditForm.aspx). You can verify after open the site with SharePoint designer.
2. Try to recreate the missing forms. The next step is to use SharePoint designer to add these three forms as described here. However, we got server error.
3. The next step is to migrate the same list from another place using Sharegate. The migration failed and has the following message.
"Unable to create web part
'Microsoft.SharePoint.WebPartPages.ListFormWebPart': A Web Part or Web Form
Control on this Page cannot be displayed or imported. You don't have Add and
Customize Pages permissions required to perform this action.. In Office 365,
this is mainly caused by the farm setting "Custom Script" being
deactivated."
Now we identified the root cause.
4. Enable the "Custom Script" on the site using the script described here.
Connect-SPOService
-Url https://mycompany-admin.sharepoint.com
Set-SPOSite
-Identity https://mycompany.sharepoint.com
-DenyAddAndCustomizePages 0
5. The final step is either recreate the three forms from SharePoint designer or migrate content from another environment.
6. You might want to disable the "Custom Script" again suing Powershell.
Connect-SPOService -Url https://mycompany-admin.sharepoint.com
Set-SPOSite -Identity https://mycompany.sharepoint.com -DenyAddAndCustomizePages 1
We are not sure why the three forms lost in the first place but this blog will give you the procedure to fix the issue.
