Are you testing this with a Data Extension and do you have a field in it named 'SubscriberKey'? If not, you will need to use '_SubscriberKey' to pull in the system defined SubscriberKey value, like noted here:

https://help.salesforce.com/articleView?id=mc_es_available_personalization_strings.htm&type=5

Additionally, it doesn't appear that for your use case you should be using URLEncode(), and should be using Base64Encode().

https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-programmatic-content.meta/mc-programmatic-content/Base64Encode.htm

Something similar to the following:

%%[ var @clientkey, @clientkeyencoded, @URLunsubscribe,@enc64 SET @clientkey = _SubscriberKey SET @clientkeyencoded = Base64Encode(@clientkey) SET @URLunsubscribe = Concat("http://example.com/unsubscribe/?sk=",@clientkeyencoded) ]%%

%%=v(@URLunsubscribe)=%%

Are you testing this with a Data Extension and do you have a field in it named 'SubscriberKey'? If not, you will need to use '_SubscriberKey' to pull in the system defined SubscriberKey value, like noted here.

Additionally, it doesn't appear that for your use case you should be using URLEncode(), and should be using Base64Encode().

Something similar to the following:

%%[ 
var @clientkey, @clientkeyencoded, @URLunsubscribe,@enc64
SET @clientkey = _SubscriberKey
SET @clientkeyencoded = Base64Encode(@clientkey)
SET @URLunsubscribe = Concat("http://example.com/unsubscribe/?sk=",@clientkeyencoded)
]%%

%%=v(@URLunsubscribe)=%%