Well I could be wrong, looks like most have confirmed that Barnabus is not scripture. As far as their criticism regarding the sabbath, in no way did it say, the sabbath was changed to Sunday. I believe it said, we keep the 8th day as the day of gladness. Why can't you be glad on the 8th day? Don't see anything wrong with that. Maybe I'm stretching it out to much, but if you keep the Bible in the back of your mind, Barnabus has some interesting points.
There are more doctrinal errors according to the site, but maybe it's just a wrong perspective?!?! Maybe it's just a matter of trying to make it fit, rather than trying to find errors? I don't know for sure. I really liked the Epitstle of Barnabas, so maybe it takes some time to understand it for what it is?!?!
Text below is a direct quote from website.
Doctrinal Errors
However, the doctrinal errors contained in the Epistle of Barnabas are quite striking, showing that this book could not possibly be “inspired” by God. We are setting forth below some of its major errors:
Error #1:
The Epistle of Barnabas alleges that the Seventh-Day Sabbath has been replaced by the “eight day” Sunday, on which “Jesus rose from the dead.”
Quoting from the 1885 translation of the “Apostolic Fathers, Charles H. Hoole,” we read in chapter 15, verses 8 and 9:
“(15:8) And, further, he saith unto them, Your new moons and your sabbaths I cannot endure. See, now, what he meaneth. The sabbaths, that now are, are not acceptable unto me, but that which I have made is, even that in which, after that I have brought all things to an end, I shall make a beginning of the eighth day, which thing is the beginning of another world. (15:9) Wherefore we keep the eighth day as a day of gladness, on which also Jesus rose from the dead, and after he had appeared ascended unto heaven.”
Nowhere does the Bible teach that the Sabbath was replaced by Sunday; nor, that Sunday is the “eighth day”; nor, that Christ rose from the dead on Sunday. For more information, please read our free booklets, “Jesus Christ–a Great Mystery,” and “God’s Commanded Holy Days.”
https://www.eternalgod.org/q-a-7171/