Another problem with the English sentence is that "prayer beads" is plural and "is" is singular, so the combination does not sound right. Maybe "My string/strand of prayer beads is very long." or "My rosary is very long." would fix this issue.
Why not "Tasbihi yangu ni ndefu mno? I marked the two sentences and it was wrong
Report it. Elsewhere I've only seen mno used for "too (much)" but in this course, it's been taught more or less as a synonym of sana, so something needs to be changed in any case.