{thassa} states that she requires a certain amount of devotion for her to become a creature, does this mean she can't attack/block before the required devotion is available?
That is correct. Until the devotion is met the card is an enchantment.
RESOLVED
To clarify...
While it's an enchantment it is not a creature, and vice versa. When it's not a creature it is affected by things that refer to enchantments but not creatures, and cannot attack or block.
Wait so thassa isnt both when devotion is met? I assumed the gods stayed enchantments.. O.o
Thassa is always an enchantment.
She is not always a creature.