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If after installation of Google Sheet Add-On, the plugin will not visible after opening the sheet again then follow the below steps. 

1. Click on Extension, hover to Add-ons, and then click on Manage add-ons.

2. Click on the three dots.

 

3. Select use in this document option.


4. Click on Extension, hover to Add-ons, and then click on use in this document.


After this Add-On will be available in your extensions for new and old Google sheets