I’ve uploaded many photos to Adobe Stock, but I keep getting the “Similar Content” rejection. How can I fix this problem? What should I change in my images or workflow to avoid similar-content rejections? I’d really appreciate your advice.

Rejection due to similar content applies not only to the asset's actual content but also to its title and keywords.
If a large number of assets with the same titles or keywords are already registered, your asset may not appear in search results, preventing sales.
You need to assign distinctive titles and keywords that reflect the asset's unique characteristics, or carefully consider the theme itself when creating the asset.

https://helpx.adobe.com/stock/contributor/submit-your-content/submit-distinct-content/distinct-content-submission-best-practices.html