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.