HEIC isn't a technical issue, however, when you upload to Adobe Stock, the file format is JPEG. The conversion to JPEG therefore may introduce more compression, so the quality can degrade more. The smartphone also uses a smaller sensor, so this in itself isn't good for large prints. Fine for onscreen viewing and small prints, but not good for large prints. The lighting conditions have to be good as well.
HEIC format isn't widely used yet, (as it's an Apple format) so it might be better to set the format to JPEG - on your phone. However, JPEG takes up more space and you will still get compression artifacts. The way to avoid this is to take photos in raw. It makes it easier to enhance the photo, and from that you can convert to JPEG.
In addition to what @@MNiessenPhoto said, there is also a slight magenta in the sky that needs to be removed. The branch in the lower left needs to be removed. You also need to sharpen the photo and do some noise reduction. Tip: Lanscapes with at least F/13 to get everything in focus.