This isn't an Apex REST error as your title suggests, but more of a Visualforce markup error.

Looks like you have your attributes a bit mixed up.  Action should point to a method in your apex class, that gets kicked off as the page loads.  I don't see any method named "AccountToFACustomer" in your class.  Instead, it looks like you want to use the "AccountToFACustomer" as an extension class.

You'll need to do a few things.

1) Add an extension controller to your class like I did around line 3 in the example below:
 

public with sharing class AccountToFACustomer {

    public AccountToFACustomer(ApexPages.StandardController sc){
    }
    
    public class Response{
        public integer code {get; set;}
        public String body {get; set;}
        public boolean success {get; set;}
        public String errorText {get; set;}
        
        public Response (integer code, String body){
                this.code = code;
        		this.body = body;
        		this.success = (code == 200 || code == 201);
        }
    }
	
    public class customerResponse{
        
        public string uuid {get; set;}
        public string link {get; set;}
        public string location {get; set;}
    }
    
    public Response CreateCustomer (string description){
        
        Response resp;
        
        string endpoint = 'https://apistaging.website.net/';
        string token = 'Token XXXXXX';
        string method = 'POST';
        
        HttpRequest req = new HttpRequest();
        HttpResponse res = new HttpResponse();
        Http h = new http();
        
        req.setEndpoint(endpoint);
        req.setMethod(method);
        req.setHeader('Autorization', token);
		req.setHeader('Content-Type', 'application/json');
        req.setHeader('Accept-Type', 'application/json');
        
        req.setBody(
            '{"name":"'+ Account.Name +'",' +
            '"email":"'+ Account.Email__c + '",'+
             '"phone":"'+ Account.Phone +'",'+
             '"website":"'+ Account.Website +'",'+
             '"location":{'+
                 	'"name":"Account No: '+ Account.Sage_ID__c +'",'+
                    '"streetName":"'+ Account.BillingStreet +'",'+
                    '"locality":"'+ Account.BillingCity +'",'+
                    '"postcode":"'+ Account.BillingState +'",'+
                    '"country":"'+ Account.BillingCountry +'" }}' 
        );
        try{
            
		res = h.send(req);
        resp = new Response(res.getStatusCode(), res.getBody()); 
            
            if (resp.success) {
                customerResponse custResp = (customerResponse)JSON.deserialize(res.getBody(), customerResponse.class);
            }
            
        }
        	 catch(System.CalloutException e) {
   				 System.debug('Callout error: '+ e);
           		 return resp;
        }        
        return resp;   
    
    }
}

And 2) Update your VF page to use the extension instead of an invalid action:
 

<apex:page standardController="Account" extensions="AccountToFACustomer" >
    
</apex:page>