Is iOS 14.4 installed on your iPhone?
Is watchOS 7.3 installed on your Watch?
If not, first update your devices(s).
Make sure your Settings > General > Language & Region are set correctly.
FYI:
- Blood Oxygen app measurements are not intended for medical use, including self-diagnosis or consultation with a doctor, and are only designed for general fitness and wellness purposes.
- The ECG app and irregular rhythm notification require the latest version of watchOS and iOS, and are not intended for use by people under 22 years old. The ECG app is available on Apple Watch Series 4 or later (not including Apple Watch SE). The irregular rhythm notification is not designed for people who have been previously diagnosed with Afib.
Still no joy, the Watch may be imported from overseas.
Check iPhone's Watch app > My Watch > General > About > Model > tap here to see number starting with A????
Compare it with this link => Identify your Apple Watch - Apple Support