Lutherans cannot take the Eucharist at an RC church, nor can an RC accept the Eucharist at one of our confessional Lutheran churches.
That is because we are divided on the real presence. We both believe in it, but disagree as to “how” the real presence is well present. RC goes with Transubstantiation, that only the accidents of bread and wine remain post consecration. Lutherans go with sacramental union, that bread wine, and Jesus body and blood are present, but without the substance/accidents division.
We are also divided on other things, such as justification, sanctification, purgatory, the power and primacy of the pope etc. Taking communion means you are in communion with that particular church. Communion means you are in unity, that is total agreement with all doctrine. Since Lutherans and Catholics are not in total agreement, it’s better that our tables remain separate for now. It may even be that our tables remain separate until the Second Coming. That would be unfortunate, but I am not willing to compromise one bit on any issue of doctrine. And I would never want anyone else, especially the RC to compromise any of their doctrine for the sake of unity. That would be doing both of us a disservice.