Copying and pasting in PowerPoint Online differs from copying and pasting in the PowerPoint desktop application because of web browser technology limitations. If you are using the Firefox browser to access the PowerPoint Online, see Copy and paste slides using Firefox.
Select the slide you want to copy, and then on the Home tab, click Copy.
Still in the thumbnail pane, right-click where you want the copied slide to be pasted, and then click Paste.
Note: If you are prompted for access to the Clipboard, click Allow Access.
Copy and paste slides using Firefox
You’ll need to use keyboard shortcuts to copy and paste slides using the Firefox browser.
In the thumbnail pane, select the slide you want to copy, and on the keyboard, press Ctrl+C.
Still in the thumbnail pane, go to where you want to paste the slide, and on the keyboard, press Ctrl+P.