Note that I'm linking to the UA because it is freely available and the content for this spell is almost identical to its released version in Tasha's Cauldron of Everything.
This is a question that carries within it many questions.
From the Bestial Spirit Stat block:
From the Summon Bestial Spirit spell:
The creature physically resembles an animal of your choice that is native to the chosen environment...
It appears to be both a Beast and a Creature. However, it lacks a CR.
Is there an example of a Beast with no CR?
Yes, DMG Page 169, the Giant Fly (created by the Ebony Fly magic item) stat block lacks a CR entirely.
Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before.
Your druid level determines the beasts you can transform into, as shown in the Beast Shapes table. At 2nd level, for example, you can transform into any beast that has a challenge rating of 1/4 or lower that doesn't have a flying or swimming speed.
Would a creature with no CR be considered "less than" the Druid's Wild Shape max CR, or "invalid" for Wild Shape?
If this could be done, given the Bestial Spirit stat block has calculations based on spell level, is there a RAW or RAI way the stat block of the Druid would be determined by?