From the error, it is obvious that, you are using some custom extension in the footer and there is a helper call like this somewhere inside the footer section.
Since that extension is wrongly confgiured the helper section, Magento couldn't find the relevant helper corresponding to this call. So you should find the extension and add this inside it.
This is all we can say about this error right now. To get more info, you should put more details to the question
[Namespace]_[Module] should replace with the correct extension name