It depends on what is meant by panentheism, since the concept is associated with many doctrines about God and creatures, very different from each other.
In reality, the use of the term panentheism is almost always associated with non-Christian realities (for example Hinduism, New Age, Gnosticism) so if we said “St. Francis is a panentheist” we could create confusion.
However, a use of the word compatible with Christian doctrine cannot be ruled out in principle, as perhaps happens in the Orthodox Church (to deepen, I am ignorant).
It is clear that the strong awareness of the presence of God in all his creatures, which St. Francis continually manifests, deserves special attention, and that an appropriate term can be sought.