Of course a teenage parent can adequately do these things. The problem is that it will be harder for a teen parent to do it. There is nothing that a teen parent is completely incapable of doing. However, teen parents' relative youth makes it harder for the average teen to do a good job raising a child. This varies, of course, from individual to individual.
The problem with being a teen parent is that you are not yet completely grown up (if there is such a thing) and yet you are trying to raise a child. Teens tend to have, for example, less of an ability to judge the long-term impacts of their actions. They also have a harder time controlling their emotions than adults do. (Again, these are generalizations that are not true for all teens.) Because of this, it is harder for teens to be parents because so much of parenting involves keeping control of one's emotions and trying to understand what is in a child's best long-term interests.
Because teens are not yet completely mature in ways like this, it is not easy for them to do these sorts of things. However, it is possible and we should not assume that every child of a teen parent will have trouble developing.