compiler error no match for 'operator=='

#include<iostream>
#include<string>
#include<fstream>
#include<iomanip>
#include<stdlib.h>


using namespace std;

enum CCType{ VISA, MASTERCARD, INVALID };
int const LINEMAXIMUM = 60;
int const CCNUMBERLENGTH = 16;

CCType validate (string aCreditCardNumber);

int main ()
{
	ifstream myIn;
	string aCreditCardNumber;
	float amount;
	string filename;
	string VISA[LINEMAXIMUM];
	string MASTERCARD[LINEMAXIMUM];
	string INVALID[LINEMAXIMUM];
	float VISAamt[LINEMAXIMUM];
	float MASTERCARDamt[LINEMAXIMUM];
	float INVALIDamt[LINEMAXIMUM];
	CCType type;
	CCType Cardtype;
	
	int l=0;
	int m=0;
	int n=0;
	
	
	cout <<"What is the name of the file"<< endl;
	cin >>filename;
	myIn.open("dataset.txt");
	
	while (filename!="dataset.txt")
	{	
	cout <<"File does not exist"<<endl<<"What is the name of the file"<<endl;
	cin >> filename;
	}
	
	while (myIn)
		
	{  
		myIn >> aCreditCardNumber >> amount;
		//Cardtype = validate(aCreditCardNumber);
		if (Cardtype == VISA)
		{
			VISA[l]=aCreditCardNumber;
			VISAamt[l]=amount;
			l++;
		}
		if (Cardtype == MASTERCARD)
		{
			MASTERCARD[m]=aCreditCardNumber;
			MASTERCARDamt[m]=amount; 
			m++;
		}
		if (Cardtype == INVALID)
		{
			INVALID[n]=aCreditCardNumber;
			INVALID[n]=amount;
			n++;
		}
		
	}
	myIn.close();
	

	
	
	
	
	
	
	
	
	
return 0;
}	
CCType validate (string aCreditCardNumber)
{	CCType type;	bool checknum = true;
	int sumi=0;
	int sumj=0;
	int ccdigit=0;
	int jnum=0;
	int sumij=0;
	int credit[15];
	
		for (int i=1; i<=15; i=i+2)
			{ sumi= (aCreditCardNumber[i] - '0') + sumi;
			}
		for (int j=0; j<=15; j=j+2)
		{	jnum=(aCreditCardNumber[j] - '0') * 2;
			if (jnum<10)
			{	sumj= jnum+ sumj;
			}
		
			 else  sumj=((jnum%10)+1)+sumj;
		}	
			sumij=sumj+sumi;
			if (sumij%10!=0)
				checknum==true;
			if (sumij%10 !=0 || aCreditCardNumber.size()>16)
			{	checknum==false;
				
			}
			if (checknum==true)
			{
				if (aCreditCardNumber[0]=='4')
				type = VISA;
		
				else if ((aCreditCardNumber[0]=='5') && (aCreditCardNumber[1]=='1' || '2' || '3' || '4' || '5'))
					type = MASTERCARD;
				
			
				else type = INVALID;
			}
return type;
}