You need to make sure that the values in the attribute table is numerical values. Excel sometimes tricky, you can see numerical values in the attribute table but Excel treats the values as text. Therefore, you need to convert the text digits into numbers or numeric by selecting the numerical column and from the
Format cell change it to
Or easier, save the file as CSV, and it should handle the values as numbers not text.
Also, you need to save the the polyline as new polyline shapefile after joining the shapefile with the CSV to make the join permanent, so you can select it when doing buffer analysis.