I assume you are using an ancient version before v1.2 then. If so, please update sfos-upgrade first, before upgrading SailfishOS with it. The current (since v1.2) hard free space limit for non-BTRFS root filesystems is 0,5 GB, which is pretty tight when performing an upgrade to a different version than the directly following one (even might be too tight then). That is the reason why a warning is shown when below 0.75 GB free space on a non-BTRFS root filesystem.