Redshift isn't showing up in my toolbar, what's wrong?

Firstly, please check that your machine matches the required specifications in the link below.

https://www.maxon.net/en/requirements/redshift-requirements

Redshift still currently uses the old method of installing plugins which is to create a "plugins" folder in your Cinema 4D installation path (C:\Program Files\Maxon Cinema 4D R20\plugins on Windows, or Applications/Maxon Cinema 4D/plugins on Mac) and install Redshift there. Before the release of R20, this was the usual method of installing plugins.

But with the release of R20, MAXON created a new method of installing plugins that does not require you to use this old method. Although Redshift, by default, will try to use this old method, you can force it to use the new method instead; here's how to do that:

First, if you've already installed Redshift, uninstall it.
Next, run the Redshift installer again. This time instead of installing to the default location, create a new folder on your computer and tell the Redshift installer to install Redshift there:
Install Redshift to that folder. The Redshift installer will warn you that no Cinema 4D installation was detected in that folder; this is fine. Click "Yes" and proceed with the installation:
Once the installation is complete, If you have your Redshift license through Redshift itself, you will need to run the licensing tool to license the plugin first. If you have a Cinema 4D + Redshift bundle subscription you can just quit the installer without running the Redshift licensing tool.
Once you're done re-installing Redshift, launch Cinema 4D and go to the Edit menu and choose Preferences. In the Preferences window go to the Plugins section and click the Add Folder button near the bottom:
Select the folder you created and installed Redshift to earlier:
Restart Cinema 4D and you should see Redshift appear normally: