Ultimately, my pictures haven't been accepted by Adobe Stock. The technical requirements are not understandable to me; could you help me understand why? I would like to improve my editing process. I edit my pictures with the Darktable app.

The technical requirements are not understandable to me...

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Start with the 4-part Photography section in your Contributor User Guide below.

1. https://helpx.adobe.com/stock/contributor/submit-your-content/submit-photos/photo-submission-overview.html
2. https://helpx.adobe.com/stock/contributor/submit-your-content/submit-photos/technical-legal-requirements-photo-submission.html
3. https://helpx.adobe.com/stock/contributor/submit-your-content/submit-photos/photo-editing-best-practices.html
4. https://helpx.adobe.com/stock/contributor/submit-your-content/submit-photos/submit-photos.html

If you’re new to photography, consider enrolling in some courses. 

Good luck.

Are you using a fixed focus camera? 

I ask because the majority of this image is not fully focused. 

Reasons for Refusal -- Quality & Technical Standards.

https://helpx.adobe.com/stock/contributor/content-moderation/quality-technical-standards-reasons-content-refusal.html

Lina, one of the most exciting things about shooting stock is that you can challenge yourself without the pressure of a real life assignment. It’s not that your photos are bad, they simply lack something special that would interest buyers. Imagine adding human elements to the same scenes you’re sharing, then adding plan every detail and image all the ideas you’ll do before the shoot day. Make it a goal to have 25 final photos so shoot no less than 400.  
I understand if you find it difficult or intimidating, but doing it you’ll discover how much you really love photography and how far you want to take it.

I’d love to see what you create!

Do it!

The image size is too small to see clearly.
Please upload an image that is the same size as the one you originally posted.
If the file size exceeds the limit, you can use external storage services like Google Drive.

Regarding the flower photo, the framing is a concern.
Before taking the shot, consider a composition that presents the flower beautifully.

It is fine if it was included intentionally, but generally speaking, people do not want to look at withered flowers.

Regarding the bird photo, the framing appears tilted.

They all appear soft and out of focus to me. Also, there are over 77 million results for flowers on adobe stock. Even if accepted, they would be unlikely to appear in the first 100 pages of the search results. Also, the gate in the third image with the bird is slanted vertically.