Repair cracks in floor

First of all you will need to investigate the problem properly which means you need to check if it’s a structural problem causing the cracks (weak joists, not properly screwed ply wood/floorboards, thin plywood/floorboards etc).If so than you need to deal with it as per regulations and I can assure you that the problem will be solved. If it is not a structure problem as mentioned above than for levelling up a floor use self levelling compound.But before you do that make sure the surface you applying the self levelling compound to is clear of any dust/dirt.After you have cleaned the surface apply a thick coat of PVA. Give it a bit of time for that to dry than is ready for the self levelling compound. Once you have levelled your floor I would advise you to give it another coat of PVA on top of that so it seals everything and gives it more strength and no cracks.I hope this would help.

It is very difficult to give an advice on how to repair a problem that it is not properly Identified, what kind of flooring ,is it concrete or just floorboards, it needs an actual assessment on the problem only then I can give any posible solution to the problem, thanks Oscar

I agree with the last writer. There is insufficient detail to give a definitive answer. Is the floor timber or screed? More details needed.

You do not repair cracks with pva or floor levelling compound .I assume your talking about a concrete floor if it’s a wooden floor you’ve got major problems.If there is a crack it’s probably the subbase has sunk .Widen the crack with a jackhammer to about 600mm then fill with fresh concrete on dpc and fit re-inforcing mesh in the concrete.