Note: Ensure swift language version of your project. Here is how you can see/check your swift language version.
You have two options as solution to your query:
If your project has Swift versio 4.0
- You should choose/download POD compatible to your project's swift language (Share me POD info and swift version, so I can provide you exact pod version version for your pod library suitable for project).
If your project has swift version below 4.0
- You need to migrate your project into Swift 4.0 (if you've not migrated it). Here is ref question and answer, how to migrate from swift (below) <4.0 to 4.0.
According to tag added by you in your question - Swift3 is your current project language version and pod 'Cosmos', '~> 12.0' is supporting swift 4.
pod 'Cosmos', '~> 12.0'
Here is list of previous release supporting Swift version below 4.0.
https://github.com/evgenyneu/Cosmos/releases
Try previous release of cosmos like:
pod 'Cosmos', '~> 11.0.3'
// or
pod 'Cosmos', '~> 11.0.1'
// or
pod 'Cosmos', '~> 11.0.0
'
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