Why I cant see the battery Health ?

Batteries are consumables; they lose a little capacity every time they are discharged, then recharged. On average this works out to about a 1% loss for every 25 “full charge cycles”. As one example, if you charge the phone overnight, every night (and that is what you should do; it is a best practice), it starts the day at 100%. If it drops to 20% by the end of the day before you charge it again overnight that counts as 0.8 full charge cycles (20% to 100%), or about 24 full charge cycles per month of use. For this example your battery capacity will lose about 1% per month. Of course, if the end-of-day level is higher than 20% the capacity loss will be a little less, and if it is lower than 20%, or you charge it during the day, the capacity loss will be higher.

Once the capacity drops below 80%, or if there is a message in Battery Health that the battery is not meeting peak performance expectations, it’s time to change the battery→iPhone Battery Replacement - Official Apple Support

The absolute best way to get maximum use on a charge, as well as slow the decline of battery capacity long term is to enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight, every night. The battery will fast charge to 80%, then pause. During the nighttime pause the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern. If you enable iCloud Backup (Settings/[your name]/iCloud - iCloud Backup) the phone will back up overnight also, assuring that you can never lose more than the current day’s updates. Here's more information→About Optimized Battery Charging on your iPhone - Apple Support

You have asked two questions.

The first is why you cannot see your battery health. If you cannot see your battery health on an iPhone, then either the battery has been replaced with a battery that does not provide that information, or because the battery has malfunctioned. If you have had the battery replaced, check with whoever replaced it. If you have not had the battery replaced, check with Apple.

The second is why a battery does not remain at its original and maximal capacity, and that is because batteries will inevitably degrade and fail. The components of any rechargeable battery age. No battery lasts forever.

Milleño Said:

"Why I cant see the battery Health ?: I just want to know Why my battery cant continue to 100%"

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About Battery Degrading:

The Battery Health drops 1-3% out-of-the-box. Why has occurred is normal. Batteries degrade as they are used, eventually depleting (which you should avoid). It cannot stay at 100%, and you cannot bring it back up (increase the percentage), yourself. Note that the Battery Health percents can even increase are times, being it is all a guesstimation on Apple's part.

Why this Occurs:

Unfortunately, there is no way to know how nor when this degrade or update occurs.