Android 12 Eligibility :
Samsung Galaxy A51, initially released on December 2019 came with One UI 2.0 on top of Android 10. It got its first major update on February 2021 with One UI 3.1 on top of Android 11 in some markets with Wide roll out on March 2021. And officially Samsung has promised 3 Year Update for Samsung A5x and A7x series. So Samsung Galaxy A51 is eligible for Android 12. And with Android 12 Samsung devices have its own One UI Skin this also will be updated to One UI 4. And it should be also be eligible for Android 13 when it launches Next year. As Android 12 is still in beta and expected to release this Month (September 2021) or early next month the Release schedule for A51 is unknown. But we expect the release period to be same as last major update around March 2022 or earlier. Since Samsung is currently releasing Beta version of Android 12 for Samsung S21 series One UI 4 features list will officially release soon.
We will update the article accordingly when the schedule release. You can check update status down here for Major and security update status.
[Update 17-Nov-2021] : According to Samsung List of One ui 4 update schedule. Samsung A51 will get one ui 4 based on Android 12 update in April 2022. The update may vary according to region you can check out One UI Schedule for your region in Samsung Members App.