Installing

Office Scripts is available in Office on the Web and Desktop.
Office Scripts is enabled by default in Office 365, although you administrator could have disabled it.

Admin Center Settting

Office Scripts has to be enabled through the Microsoft 365 Admin Center.
Login to Office Online.
Select Admin and click "Show all" on the left hand side.

Expand "Settings" and click "Org settings".

In the list find the item "Office Scripts".

Click on "Office Scripts" to display a pane on the right hand side.

Check "let users automate their tasks in Office on the web" checkbox.
Click "save changes".
Open Excel Online and launch a new blank workbook.
An additional tab will be displayed called "Automate".