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.