From Mosaic on your iPhone, when you click on “Check for Update” Mosaic actually instructs your dCS platform to contact the dCS Firmware hosting Server on the Internet.
But before downloading the new firmware, your dCS platform downloads a manifest in order to compare the version held on the dCS Firmware Server against the current firmware on your dCS platform - in case it already has the latest. If the versions are the same, then Mosaic will display “No firmware update”. If the Firmware on the Server is newer, then Mosaic will show the version available for update.
In your case, your dCS has a problem with that first step of downloading the manifest in order to compare firmware versions.
There could a range of reasons why that’s happened - Problems with your Internet. Problems with reaching the dCS Firmware server (they’re actually hosted on CDN caching servers), etc. My suggestion, just to be sure, you should reboot your dCS using the rear power button, and try the update process again.