Why can't Ukraine freely trade with both the European Union and Russia?

Ukraine jpined and signed the European Union Association Agreement

The agreement commits Ukraine to economic, judicial, and financial reforms to converge its policies and legislation to those of the European Union.

Ukraine commits to gradually conform to EU technical and consumer standards.The EU agrees to provide Ukraine with political and financial support, access to research and knowledge, and preferential access to EU markets.... so in doing that it gives the Ukraine millions in financial support and a huge market to sell to with limited taxes/custom charges so it also legally restricts some trading with other countries outside the EU

They can. Who told you they can't?