S184 is section 184 of the housing act 1996.
It's the law that says if a local authority has reason to believe you are homeless, or at risk of becoming homeless, they must find out whether you are eligible for assistance and what housing duties they owe you.
They have a duty to inform you of the outcome of their inquiries and give reasons and this is the 'S184 letter'
It probably won't have 'SECTION 184' written across the top. It might be called something like, 'Our decision on your homelessness application' and it will probably say something like, 'Dear Ms Heroine, Further to your recent application for homelessness assistance we have concluded our inquiries in accordance with our duties under S184 of the Housing Act bla bla bla ...'
It's important to work out where you are in the process and what decisions have been made - whether you've had this decision letter, whether they decided you were homeless or just at risk of becoming homeless, what duties they have decided they owe you, whether you still have time to request a review if you disagree with the decision, whether you have been given a personal housing plan and if you have, what you have agreed to do. It's possible that you have agreed to look for a suitable private rented property as part of your plan.
If they have decided that you are only at risk of becoming homeless, and if they decide you are not complying with your housing plan, then they can stop helping you altogether until you are actually properly homeless.
Read today's email carefully - are you required to look for properties or is it a helpful suggestion? If it looks like a requirement then go through the motions - spend X amount of time browsing the sites (you never know), then put in writing 'I have spent x amount of time on these sites but none of the properties were suitable.' Tell them again what features a property must have to be suitable for your DD and for you.
Dig out past correspondence - you're looking for a homelessness decision letter and a personal housing plan. If you never received these or you just can't find them, ask the council for copies. You are legally entitled to copies of this paperwork. Once you have these documents you can work out what your options are.
Call Shelter for advice and also take up Jude's suggestion of contacting CLA. Your local law centre or CAB might be able to provide a legal aid appointment with a specialist housing solicitor.
Useful basic guide to the whole process here:
england.shelter.org.uk/housing_advice/homelessness/guide/homeless_get_help_from_the_council/how_the_council_can_help