iPhone doesn't keep charge

Check your battery health.

Read this support article about iPhone battery and performance to learn about the battery health feature and what is considered normal performance --> iPhone battery and performance - Apple Support

To see the feature go to: Settings > Battery > Battery Health

"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge." A degraded battery may display the notice: "Your battery’s health is significantly degraded. An Apple Authorized Service Provider can replace the battery to restore full performance and capacity." If it is not at or above 80% "normal", consider contacting Apple about a battery replacement. --> iPhone Battery Replacement - Apple Support

According to the above quoted information, you could be losing almost 2% per month the entire first year before Apple would deem this to be unusual battery behavior and merit a warranty replacement.

Also read:

- Apple’s support article about batteries draining too quickly —> If the battery in your iPhone or iPad drains too quickly - Apple Support

- This post by ASC contributor Lawrence Finch which has a very user-friendly explanation of battery features --> Bettery health maximum capacity - Apple Community

Go to this web site for information about how to request battery service for your iPhone: iPhone Battery Replacement - Apple Support

For cost and availability, enter your model information in the "Get an estimate" box at the bottom. (It is best that you do this since sometimes contributors here forget this is an international forum and not everybody asking questions lives in the contributor’s country.)

For instructions on how to get your iPhone, iPad, or iPod touch ready for service click here --> Prepare your iPhone or iPad for service - Apple Support

What did the technician at the Apple Store or authorized Apple service shop say when you took the phone in to have it checked and tested?