Start with the file, not the app
Open Files and check the extension and file size. A file ending in .epub should contain an EPUB package; a zero-byte or unusually tiny download may not have finished. Download the book again from a trustworthy source before changing phone settings.
Try a different EPUB that you know works. If both files fail, the reader or its file permissions may be the issue. If only one fails, focus on the problem book rather than repeatedly reinstalling the app.
Tell an EPUB problem from a DRM problem
An EPUB sold through a bookstore may be encrypted. The fact that the file has an EPUB extension does not mean every reader is licensed to decrypt it. If the seller requires a specific app or account, open the title there. Do not upload paid books to random online converters.
If it opens but looks wrong
Missing chapter breaks, strange characters and misplaced images can mean poor source formatting or unsupported EPUB features. Try the original file on a second device or reader. Check whether your app has a setting to respect publisher formatting or override font styles. Fixed-layout EPUBs may behave differently from normal novels.
Fix an import permission error
On newer Android releases, a reader may use the system file picker to access one selected document rather than all storage. Import from inside the reader instead of assuming it will see every folder automatically. Move a copy of the book to Downloads if it lives in a restricted app folder.
Keep the original download untouched and work on a copy. If none of these steps helps, note the exact error message and whether the failure occurs during import, opening or page display; these point to different causes.
To check whether the problem follows the file, try a known-good, DRM-free EPUB in Readream Basic. If one book opens and another does not, compare their downloads before changing more phone settings.