The exact error:
Minimum API Level Not Supported on Requested VR Device
Cardboard Requires a Minimum API Level of 19
Though in the Android SDK Manager, it says I have revisions 25.2.5, 25.0.5, and 25.0.3 installed.
You have to change this Minimun API Level from the Editor.
Go to Edit --> Project Settings --> Player --> Other Settings --> Identification --> Minimium API Level --> Android 4.4 'Kit Kat' (API Level 19) or anything above.
Also change Target API Level to Automatic (highest installed).
If this did not work, make sure to download API Level 19 (Android 4.4.2) or any level above from the Android SDK Manager. After that repeat what's in this answer.