Which version of Revit are you using?
eTransmit is installed by default with Revit 2018 & 2019 but it still appears under the Add-Ins tab. For older versions you would have to download and install per version.
For 2019 It should be available under the Add-Ins tab as part of the core install.
I wonder if this was somehow skipped during the installation of Revit. If you Right Click on Revit 2019 under Programs and Features in the Control Panel, do you have an option for Uninstall/Change (or something similar)? If so, you might need to add this feature to your installation. Not sure if this option works to find what is not installed with Revit but it does exist for other programs.
Or, you might want to check your install folder/media to see if eTransmit is available there for install.
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