Content Editor Webpart not showing up in SharePoint Online
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Problem :

Solutions :

1. Go to SharePoint Admin Center -https://XYZCOMPNAY-admin.sharepoint.com
2. Go to Settings --> Click on Can't find the setting you're looking for? Go to the Classic Settings Page.
3. Allow Custom Script 

It will be activated in 24 Hours.

If we want to activate immediate custom script , We have to enable it from Sharepoint Online Management Shell.

Activate Custom script from Sharepoint Online Mangement Shell :

Hope Sharepoint Management Shell already installed in the system.

PS C:\Windows\system32> Connect-SPOService

cmdlet Connect-SPOService at command pipeline position 1
Supply values for the following parameters:
Url: https://XYZCompany-admin.sharepoint.com
PS C:\Windows\system32> Set-SPOSite https://XYZCompany.sharepoint.com/sites/Development -DenyAddAndCustomizePages 0
PS C:\Windows\system32>

Sharepoint Online Management Shell
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SharePoint online Management shell is a windows power shell module loaded with the share point libraries for share point online , and hosted on office 365 , it helps administrators to manage users , sites and site collections .

Powershell is highly effective and easy to use for managing SharePoint, Everything that can be done on SharePoint central administration can be done using power shell. 

As SharePoint hosted on office 365 , there have been limited options for SharePoint administrators to use power shell. How ever with the recently released SharePoint online management shell , it will be easy to manage SharePoint online from any remote computer. 

Not every command is available in SharePoint online compared with On-Premises , But it's a certainly a good start.

Install the SharePoint Online Management Shell

After installing the SharePoint Online Management Shell , To open the SharePoint Online Management Shell command prompt, from the Start screen, type sharepoint, and then click SharePoint Online Management Shell.

To connect with a user name and password

1. 1.    Fill in the values for the $adminUPN and $orgName variables (replacing all the text between the quotes), and then run the following commands at the SharePoint Online Management Shell command prompt:

PS C:\Windows\system32>   $adminUPN="sampleuser@xyzcompany.onmicrosoft.com"

PS C:\Windows\system32>  $orgName="xyzcompany"

PS C:\Windows\system32>  $userCredential = Get-Credential -UserName $adminUPN -Message "Type the password."

PS C:\Windows\system32>  Connect-SPOService -Url https://$orgName-admin.sharepoint.com -Credential $userCredential

2. When prompted with the Windows PowerShell credential request dialog box, type the password for the SharePoint Online SharePoint administrator account.

Ribbon missing in Share point online Pages to add custom web part :
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you can not see the ribbon if you create modern page or site page  in share point online.

Solution:

Create Webpart page or wiki page to see the the ribbon and Browser/page buttons