You can activate Crawling Barrens's second ability even if it's already a creature.

Counters on Crawling Barrens remain on it when it stops being a creature. If it becomes a creature later, they'll apply to it.

Unless Crawling Barrens is already a creature, its second ability causes the +1/+1 counters to be put onto a noncreature land. Abilities that apply or trigger when counters are put on a creature you control won't do so.

If this becomes a creature but you haven't controlled it continuously since your most recent turn began, you won't be able to activate its mana ability or attack with it that turn.