If you are making changes to the IP settings of a device and setting it from DHCP to Manual, the OK button will be greyed out by default
Before:
After:
The button will continue to remain greyed out even if you populate the IP address, mask and gateway settings.
This is because the DNS settings are set to 0.0.0.0 which is invalid.
You need to enter either the local DNS server IP , or a global DNS such as Google's DNS and the OK button will then allow you to save the settings