If you're having trouble posting to pixiv from Pastela, check the following.
・Have you spent a long time on the posting screen?
Staying on the posting screen too long can lead to errors.
Try posting again from a place with a good network connection.
・Are you logged into the same pixiv account on both Pastela and pixiv?
To post artwork, you need to be logged into the same pixiv account on both Pastela and pixiv.
If the issue persists, the problem might be with your environment. Please try the following:
・Post again from a place with a good connection.
・Restart your device.