can't delete file from external storage in android programmatically

Using ContentResolver to delete media files is wrong and provides many problems for the user.
You can not delete a file on the sd-card simply by deleting its information from the ContentResolver on Android versions greater than Jelly Bean(4.3).
It works only on Android versions prior to KitKat(4.4).
That's why the Android team provided DocumentProvider.

Why contentResolver.delete(...) is wrong?
1. Fills up the sd-card
When you try to delete a media file on the sd-card by the ContentResolver on Android versions greater than 4.3, the actual media file will remain untouched because the contentResolver.delete(...) approach only removes the information (name, date, path ...) of the media and you will end up having unregistered media files on your sd-card which ContentResolver has no idea about their existence anymore and that's why you couldn't see them in your gallery and you think they've been deleted with this approach while they're still there and fill up the sd-card each time the user tries to delete a media file on the sd-card.

2. Media files (Images, videos, gifs ...) will come back to the gallery
There are many apps out there especially gallery and file manager ones that will find these unregistered media files and will add them to the ContentResolver again as of their normal behavior while the user assumes his/her unwanted media files are gone. Sure no user wants his/her assuming deleted images or videos show up in the middle of a demonstration.

So, what's the correct approach to remove media files on the sd-card?
Well, this has already been answered here with the use of DocumentProvider.

public static boolean delete(final Context context, final File file) {
    final String where = MediaStore.MediaColumns.DATA + "=?";
    final String[] selectionArgs = new String[] {
            file.getAbsolutePath()
    };
    final ContentResolver contentResolver = context.getContentResolver();
    final Uri filesUri = MediaStore.Files.getContentUri("external");

    contentResolver.delete(filesUri, where, selectionArgs);

    if (file.exists()) {

        contentResolver.delete(filesUri, where, selectionArgs);
    }
    return !file.exists();
}

From Android 4.4 onwards, you can't write to SD card files (except in the App directory) using the normal way. You'll have to use the Storage Access Framework using DocumentFile for that.

The following code works for me:

private void deletefile(Uri uri, String filename) {
    DocumentFile pickedDir = DocumentFile.fromTreeUri(this, uri);
    try {
        getContentResolver().takePersistableUriPermission(uri, Intent.FLAG_GRANT_READ_URI_PERMISSION | Intent.FLAG_GRANT_WRITE_URI_PERMISSION);
    } catch (SecurityException e) {
        e.printStackTrace();
    }

    DocumentFile file = pickedDir.findFile(filename);
    if(file.delete())
        Log.d("Log ID", "Delete successful");
    else
        Log.d("Log ID", "Delete unsuccessful");
}

where filename is the name of the file to be deleted and uri is the URI returned by ACTION_OPEN_DOCUMENT_TREE:

private static final int LOCATION_REQUEST = 1;

private void choosePath() {
    Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT_TREE);
    intent.addCategory(Intent.CATEGORY_DEFAULT);
    startActivityForResult(intent, LOCATION_REQUEST);
}

@Override
public void onActivityResult(int requestCode, int resultCode, Intent resultData) {
    if (requestCode == LOCATION_REQUEST && resultCode == Activity.RESULT_OK) {
        if (resultData != null) {
            Uri uri = resultData.getData();
            if (uri != null) {
                /* Got the path uri */
            }
        }
    }
}