Updating to the latest version resolved this issue for me.
@johnpool3 What version currently you are using, What can I see form pod spec It still point to 4.0
@techmehra support for swift 4.2 and Xcode 10 added since version 3.3.0
@gcharita I am also using the version 3.3.0 but still getting this

How can I upgrade to 3.3,
pod 'ObjectMapper', '~> 3.3'
pod install
Analyzing dependencies
[!] CocoaPods could not find compatible versions for pod "ObjectMapper":
In Podfile:
ObjectMapper (~> 3.3)
my xcode version is 10
For everyone who want to make sure that they have the latest version
Podfile.lock have something like this:- ObjectMapper (3.3.0)
Podfile to have this:pod 'ObjectMapper', '3.3'
pod repo updatepod installThis is not working for me. I still have a "Convert to Swift 4.2" for only ObjectMapper. I am using 3.3, many pod update/installs/resets/etc. Seems to be an issue with this library in particular.
Swift 4.2 support has now been released
Most helpful comment
Updating to the latest version resolved this issue for me.