What is wrong with conference publications compared with journal and transaction papers? Our professor has set us to select a credible fresh paper and present it as part of our course in a computer engineering subject for an MS program in computer engineering. But, he insists that we select transactions and journals as categories to select papers from, thus he opposes conference papers. What is wrong with papers published in conferences making them probably unsuitable for such a presentation job?
There is absolutely nothing wrong with conference publications. This sounds like your professor has something specific in mind and wants his students to achieve that target. In general, publications in journals are more detailed (and deep) so would make the students work harder I suppose since there is more information to process. Having said that, some journal papers are smaller than conference papers in length too (but that's not very common at least in computer engineering)

There is also a perception amongst a good number of academics that many conference publications are just small/quick pieces of work submitted hastily and later followed up with a journal publication. Again, this may depend on the conference itself. For example, the flagship conferences in your area would feature very good papers compared to a localised regional conference. So this may be a concern of your professor that his students may end up selecting a paper that is not of a good quality. In the end, only your professor can answer that.