If you are currently using the Android native custom tabs as the browsing mode then the "Reader View" option is not available. However, you can switch to legacy webview which allows enabling reader mode.
To switch to "Legacy Webview" follow these steps:
1. Go to your profile
2. Tap the gear icon in the upper right corner
3. Scroll down to Display Options and tap Reading Mode
4. Select Legacy web view (or Android custom tabs if you are switching from Legacy web view)