1. *The computer lab is consist of 25 computers. <ungrammatical>
The word 'consist' is a verb, so it doesn't need an auxilairy 'is'. Our example 1. is ungrammatical because it has too many main verbs. Delete one of the verbs, say, 'is' and the resulting sentence is structurally grammatical but semantically awakward:
?The computer lab consists of 25 computers. <semantically awakward>
'consist of' means, composed of; made of. Labs (i.e., rooms) are not composed of or made up of computers. Rather, the lab is housed with 25 computers or it has 25 computers in it.
Below, our example 2., the verb 'is consisting' is present continuous in form: BE + ing,
2. *The computer lab is consisting of 25 computers. <ungrammatical>
With continuous verbs the subject does the action. Above, our subject 'the computers' are consisting / composing / making up. That's kind of odd, wouldn't you agree?
All the best.