RAW It can only affect allies
When the spell says that it affects allies it means just that. There is no fluff/flavor text in 5e spells so everything is considered an elaboration of the rules. Thus, the allies restriction in the first sentence is as much a rule as the proceeding text. However, the "allies" part need not be repeated in the targeting section since it was already stated.
As Jeremy Crawford has said in a Tweet:
Q: So the first line is flavor text, then? There are people who insist
every word in the rules have mechanical impact while others state that
some, like the first line of Divine Sense, are just flavor with the
"crunch" following after.
Crawford: In any piece of writing, context matters. If a rule has
multiple sentences, they're meant to be read together. For example,
the first sentence of Divine Sense is meant to be read with the rest
of the feature's sentences, which explain that first sentence.
So, when the spell says:
Your spell bolsters your allies with toughness and resolve.
and then later says:
Choose up to three creatures within range.
These statements are meant to be read together and in context as "three allied creatures within range".
DM can allow it to affect enemies without hurting anything
Now, as always a DM can interpret differently. In this case, this would have very little impact to rule that it affects enemies because the use of such a thing would be very, very niche.
