How can I fix this error that I get from setting a character of a string equal to a element of a vector

Question

I'm trying to make a program that converts decimal numbers to a certain base and then finding how many times the largest number (1-8) in that base appears.

string conv(int decimal, int base);
vector<int>base10nums;
vector<string>differentbase;

int main(){
    int num,base,start;
    cin>>num>>base>>start;
    for(int i=0;i<num;++i){
        base10nums.push_back(start+i);
    }
    for(int i=0;i<num;++i){
        string x=conv(base10nums[i],base);
        differentbase.push_back(x);
    }
    int u=0;
    int largestdigit=base-1;
    for(int i=0;i<num;++i){
        string l=differentbase[i];    //error here
        for(int j=0;j<differentbase[i].length();++i){
            if(largestdigit==8){
                if(l[j]=='8'){          //error here
                     u++;
                }
            }
            if(largestdigit==7){
               if(l[j]=='7'){
                     u++;
               } 
            }
            if(largestdigit==6){
                if(l[j]=='6'){
                     u++;
               }
            }
            if(largestdigit==5){
                if(l[j]=='5'){
                     u++;
               }
            }
            if(largestdigit==4){
                if(l[j]=='4'){
                     u++;
               }
            }
            if(largestdigit==3){
                if(l[j]=='3'){
                     u++;
               }
            }
            if(largestdigit==2){
                if(l[j]=='2'){
                     u++;
               }
            }
            if(largestdigit==1){
                if(l[j]=='1'){
                     u++;
               }
            }
        }
    }
    std::cout<<u;
}

std::string conv(int decimal, int base){
    if(decimal == 0) return "0";
    char NUMS[] = "0123456789ABCDEF";
    std::string result = "";
    do{
        result.push_back(NUMS[decimal%base]);
        decimal /= base; 
    }while(decimal != 0); 
    return std::string(result.rbegin(), result.rend());
}

In the part with a bunch of ifs, I'm trying to compare each digit in each number in base base to the largest digit. I can run the program, but I don't get the right value and when I hover over the l[j] part in VSCode, I get the error:

"inline char &std::string::operator[](std::size_t __pos)
+1 overload

Subscript access to the data contained in the %string.

Parameters:
__pos – The index of the character to access.

Returns:
Read/write reference to the character. This operator allows for easy, array-style, data access. Note that data access with this operator is unchecked and out_of_range lookups are not defined. (For checked lookups see at().)"

Any help is appreciated!