You currently use a step line chart with sub type of “Series” and you have a categorical scale.
In order to format the X axis as you require, you can use an expression to format the category labels. Under Pivot >> Data Groupings >> Categories >> General
For the Label field you can use: =FORMAT(Fields!Date, "MMM", "en-US")
However, you have multiple data points for one month and the result for the X axis will be:
Jan Feb Feb Feb Mar etc.
In this case you should be using an XY Scatter sub type and the Numeric scale. When you switch to XY Scatter sub-type, under the Pivot tab you first need to provide an X value (=Fields!Date).
Then from the Chart >> Chart Areas >> Axes >> Primary X >> Scale tab >> Numeric, you can configure the formatting:
Sub Type: Date Time
Formatting Mode: Fixed
Label Format: MMM
and set the Step Mode to custom step – 1 Month.