What does Ending (1-bar) mean? How to interpret and play this in an electronic keyboard? Does that mean the when the ending button is pressed, the default ending music should complete within a bar?
Yes, in keyboards that provide some form of accompaniment, the "ending" is a pattern that ends all musical parts in a certain duration (usually predefined by the current "style").
Depending on the keyboard, the "ending" could begin as soon as the button is pressed (by immediately changing the pattern) or at the next bar.
If the keyboard allows to define the duration of the ending (some models have different type of endings and possibly different durations) then just set that, otherwise you either choose another accompaniment, or you press the button at the right moment before what is marked in the score in order to make it end at the right time: it should stop within the end of bar 53 or the beginning of the next one at most, most importantly because there's a
rit at the end of bar 54.