Help with Filtering Using A Separate Table

Your slicer table isn’t filtering Sales, so the average uses all 6 days.

Fix: Create a measure with TREATAS

Avg Sales by Slicer =
CALCULATE(
AVERAGE(Sales[Sales]),
TREATAS(VALUES(slicer_date[Date]), Sales[Date])
)

Now the slicer dates will filter Sales, and the average for 1–5 Nov will return 3.

Alternative: just relate slicer_date[Date] → Sales[Date] directly.