The enquirer bought a single can of 5 litre paint at a DIY chain. He asked the check out assistant for a plastic bag which she placed by the tin. The assistant said she was unable to put the tin into the bag due to health & safety reasons.
Large paint tins are heavy and can be awkward to lift particularly from a sitting position. There may indeed be some health and safety considerations to take account of including considering the capabilities of the individual involved and any particular health issues. Unfortunately there is not enough information provided about the specific circumstances in this case to reach a view on whether the decision was reasonable and proportionate.
It is also difficult to understand why anyone would put a heavy paint can into a carrier bag of unknown strength rather than use the carrying handle which is normally on the tin itself.