John Gill Commentary Deuteronomy 28:40

"Thou shalt have olive-trees throughout all thy borders, but thou shalt not anoint thyself with the oil; for thine olive shall cast [its fruit]." — Deuteronomy 28:40 (ASV)

You shall have olive trees throughout your coasts
In the several parts of the land of Canaan, which is therefore called a land of olive oil, (Deuteronomy 8:8);

but you shall not anoint [yourself] with the oil ;
nor any other relations, friends, guests, as was usual at entertainments; see (Psalms 23:5) (Luke 7:38Luke 7:46); for the phrase "yourself" is not in the text. The reason why they should not anoint is, because they would have no oil to anoint with:

for your olive shall cast [its fruit] ;
before it is ripe, by one means or another, as by winds, or blasting and mildew; see (Amos 4:9) (Habakkuk 3:17).