Target Line Based on What-If Parameter

Hii armchairexpert 

You can create a dynamic target line using your What-If parameter by returning a value only for years between 2021 and the selected Target Year. For years before 2021 or after the selected year, the measure will return blank.

Try this measure:

Target Line =
VAR SelectedYear = SELECTEDVALUE(TargetYear[TargetYear])
VAR CurrentYear  = MAX('EmissionData'[Year])
RETURN
IF(
    NOT ISBLANK(SelectedYear)
        && CurrentYear >= 2021
        && CurrentYear <= SelectedYear,
    1,   
    BLANK()
)

Add this measure to your line chart as a separate series.
Now the line automatically extends from 2021 up to the chosen year (e.g., 2030) and disappears when a past year is selected.