An uninstall requires a full, proper restart of the device. Getting that to happen on a Kobo is slightly finickier than it ought to because of the lack of a simple "restart me!" button somewhere in the settings
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TL;DR: AFAIK, the least error-prone way to get that done is to power it off (hold the power button for A Good Long While(TM)). Then power it back on. If you don't see the power LED blinking, then the usual boot progress bar, it was only sleeping, not off => hold the power button longer.