This most common error is that you are naming the subnet straight out
(IE: 255.255.255.0) instead of the "equivalent prefix value" actually required ...
Method 1: Enter Subnet Prefix length instead of Subnet Mask.
Subnet Prefix length and Subnet Mask are actually different ways of specifying the same thing. That is, both specify the subnetting details of the IP address.
In the Edit IP settings window, when we have a clear look, the description asks us to enter Subnet prefix length, when we enter subnet mask instead of prefix length in this box, the error is seen.
Common Prefix Length equivalent of the subnet mask is given below
255.255.255.0 –> 24
255.0.0.0 –> 8
255.255.0.0 –> 16
For example, if the subnet mask 255.255.255.0 is given, then enter 24, which is its equivalent prefix length.
I quoted the most common issue/cause of error when manually adjusting your IP settings above ...
There's more methods and examples listed here