Yes, Pastela can be used for free.
To use cloud features, you'll need to sign up for the Free Plan.
Logging in with a pixiv account (linking it) automatically registers you for the Free Plan.
To access certain features, such as increased cloud storage capacity,
you need to subscribe to either Pastela Plus or pixiv Premium.
To learn more about Pastela Plus and pixiv Premium, see:
What is Pastela Plus?
What is pixiv Premium?
What perks do I get in Pastela with pixiv Premium?