Android 2.3.6 Gingerbread was released over a decade ago. Because modern apps require more processing power and newer security protocols (like TLS 1.2), contemporary versions of WhatsApp simply won't run on this firmware. When you search for a "working" APK, you are essentially looking for the last stable version that was compatible with the ARMv6 or ARMv7 processors common in that era. How to Install WhatsApp APK on Older Androids
That said, if you still want the process for historical or offline testing purposes, here is a safe and accurate guide. Whatsapp Download Apk Android 2.3.6 WORK