You are probably using the pre-merged version of Aric's Expansion, but that version still requires several installation steps. The pre-merging simply lets you skip moving the contents of the "Put in Strive Game Files" folder and deleting the "backup" folder.
The races are not showing up because you did not apply the AricsExpansion mod to the pre-merged version. It may tell you that it is already applied, but that is because v0.5.25 of Strive did track mod installs per instance of Strive; there is a single record for all instances of Strive on the computer. Simply make sure AricsExpansion is selected and press "Apply" again.
The first post in thread had a video of the steps, feel free to mute it. If you are using the pre-merged then it will not quite be accurate but you can start at about 1: 43 (YouTube Video). From 3:14 to 3:52 it will show how to move the "Put in Strive Game Files" folder and rename "backup" folder, SKIP this part if you have the pre-merged version.
Chances are strong that you have the wrong folder path. But if you are unable to resolve it then I recommend joining the Strive Discord (https://itch.io/t/284398/discord), where we can walk you through the installation steps.
