why 192.168.1.2/255.255.255.0 is considered as an invalid IP?
I wasn’t sure what you were asking until I viewed your question in the context of the quiz, where the question is a bit more broadly asking for CIDR blocks / IP Addresses that are not valid for a private VPC or subnet in AWS. The wording of the question is a bit awkward in the exam, but when you look at the four examples they provide, three of them are specific IP addresses, and only one of them is a CIDR block.
In the case of the address above, 192.168.1.2/255.255.255.0 can also use the notation 192.168.1.2/24. Again this is a specific IP address and falls in the range 192.168.1.0 to 192.168.1.255. Otherwise stated, IP 192.168.1.2/24 is part of the 192.168.1.0/24 CIDR Block. Within this CIDR Block:
184.108.40.206 is the network address
220.127.116.11 is the broadcast address
AWS reserve the first three IPs of any CIDR block for internal use. For this CIDR block that would include 192.168.1.1/24, 192.168.1.2/24 and 192.168.1.3/24. Since the address in the question above would be reserved by AWS it makes it invalid for use (by users) within a private VPC or subnet in AWS.
The following I think is a bit better reference than the one offered in the quiz notes: https://docs.aws.amazon.com/vpc/latest/userguide/VPC_Subnets.html
Hope this helps,