You might have seen that classic webparts are missing when you create a new site collection/tenant and try to add classic webparts like Script editor, Content editor or any search webparts in modern SharePoint site.
In the below modern SharePoint site, you can see classic webparts are missing.

In this article, you will learn how to show missing classic webparts in SharePoint online ?.
There are two approaches to show classic webparts in modern SharePoint :
To activate the custom script feature follow the below steps:
- Go to the App launcher on the left top corner.
- Select Admin > SharePoint
- Select Admin > SharePoint > Setting

- Scroll down the page and you will see the custom script and then Allow both to activate the feature

- Click Ok.
📝 Note:
By using the above steps all classic webparts will be visible but It will take nearly 24 hours to activate this feature so for the fast solution we can use our another approach: PowerShell script.
Solution 2: Using Powershell Script
Open SharePoint Online Management Shell as Admin and run below PowerShell Script.
//Replace your 'USERID' and 'DOMAIN' $adminEmail ="USERID@DOMAIN.onmicrosoft.com" $orgName="DOMAIN" //Get user credentials $userCredential = Get-Credential -UserName $adminEmail -Message "Enter password" //Connect to SharePoint Online service Connect-SPOService -Url https://$orgName-admin.sharepoint.com -Credential $userCredential //set you site collection or tenent URL Set-SPOsite https://DOMAIN.sharepoint.com -DenyAddAndCustomizePages 0
Result
After applying one of the above approach your webparts will be visible, you can see the result as below:

Sharing is Caring.
Happy Coding