1. Exegesis
Firstly, let me suggest you use the Pinkard translation which has the advantage of being recent, free, and bilingual. The text reads as follows in there:
However, in coming on
the scene, science is itself an appearance,
and as it comes on the scene, science has
not yet itself been worked out in its truth in
any extensive way. It makes no difference
in this regard whether we think of science
as an appearance because it comes on the
scene alongside other ways of knowing, or
whether we call that other untrue
knowledge its appearance.
The German sentences read:
Aber die Wissenschaft darin, daß sie
auftritt, ist sie selbst eine Erscheinung; ihr
Auftreten ist noch nicht sie in ihrer
Wahrheit ausgeführt und ausgebreitet. Es
ist hiebei gleichgültig, sich vorzustellen,
daß sie die Erscheinung ist, weil sie neben
anderem auftritt, oder jenes andere unwahre
Wissen ihr Erscheinen zu nennen.
The first sentence roughly says that insofar as science is not eternal and unchanging in its being ("auftreten" = change from not being there to being there, hence the "coming on the scene" translation), it is only an appearance. The German text also says more pointedly in the second half that science as it has come on the scene, ie. in the form it has when it has established itself as a form of knowledge, is not yet science developed and elaborated in its truth. This is also a play with equivocations hard to catch in translation since "auftreten"/"Auftreten" bears both 'the change from not being there to being there' and 'the current appearance of something' as two different connotations.
The second sentence clarifies that as long as there are "other forms of knowledge", science (German "Wissen-schaft" = that which produces (true) knowledge) has not developed into its true being and there still is a form of "mere appearance" of science. It still is faced with an "other" kind of knowledge that is not itself while it is, as a concept, literally that (ie. the only thing) which produces true knowledge. In other words: if we want to realize the concept, there actually should not be any kind of knowledge that is not scientific. Therefore, it makes no difference which side (science or "other kinds of knowledge") you name the appearance of science, the point is that science still is as mere appearance as long as there are other kinds of knowledge at all.
2. Explanation
Mind, this was a time where natural sciences just started to unravel various insights into nature, ie. scientific knowledge was apparently (sic!) incomplete since science proper "just came on the scene", historically speaking. Hegel was very interested in natural sciences and read recent books he could get his hands on, as evident by his remarks on chemistry earlier in the book.
The other factor is that in Hegel's terminological system, the idea of science (its truth) consists of Being (the totality of that which is) being in full accordance with the concept, ie. the sublation of object (being) and subject (concept) into a single Being where there is no "other" anymore. And if science is that which produces knowledge, then any knowledge other than scientific knowledge means the idea has not come to itself, ie. is not fully realized. Therefore, what he writes here is just an explanation of what it takes for science to become real/true. See also his following remarks:
But science must
free itself from this semblance, and it can
only do so by turning against it. This is so
because science cannot discard a nontruthful cognition on the grounds that it is
merely a common view of things while at
the same time assuring us both that it is
itself an entirely different kind of
knowledge and that the other kind of
knowledge amounts to nothing at all for it.
Nor can it appeal to some vague intimation
about there being something even better in
the common view. By way of that
assurance, it declares its power to lie in its
being. However, untrue knowledge equally
appeals to the same thing, namely, that it
exists, and it assures us that in its eyes
science amounts to nothing.
TL;DR
As long as there is knowledge that is not scientific, science as - per concept - literally "that which produces true knowledge" has not come to itself, ie. its idea is not real yet.