Why mine is not coming yet on my iPhone6s Plus? Mine is still IOS12.5.1

If any of the following true, your phone may not be able to update:

• Your phone does not have enough free space. Typically it should have at least 5GBs available to update.
• You have either or both Low Power Mode or Personal Hotspot enabled.
• Your phone is not connected to power and it has less than 50% charge left.
• The phone is not connected via Wi-Fi to the Internet. It won't update on a cellular-only connection.

Ref: If your iPhone, iPad or iPod touch won’t update - Apple Support

If you have a iPhone running iOs 12.5.1 then its not a 6s plus, but more likely an iPhone 6 Plus. Different devices.

iOS 12.5.1 is the current iOS version for devices incapable of updating to iOS 13 or 14. It was released a couple of weeks ago and has the fixes and updates required to secure those devices.

Because you have an iPhone 6 Plus, not a 6S Plus. It can not be updated further.