My code:
  #include <stdio.h> #include<cs50.h> #include<string.h> #include<ctype.h> #include<stdlib.h> int main(int argc, char* argv[])

{   
if (argc!=2) 
{
    printf("Error\n"); 
    return 1;
}
// argc=2
//check if key is letter

string key = argv[1];
int lenk = strlen(key);
int k = 0;

for (k = 0; k < lenk; k++)
    {
        if ((isalpha(key[k]))==0)
        {
            printf("Error\n"); 
            return 1;
        }
    }

//printf("Please insert plaintext\n");
string p = GetString();
//printf("lenk = %d\n", lenk);
int lenp = strlen(p);
int i = 0;
int kalpha = 0 ;


for (i = 0; i <= lenp; i++)
{   
    if (isalpha(p[i]))
    {
            int l = p[i];
            int s = kalpha%lenk;
            int j = key[s];
            int m1 = l + j -65;
            int m2 = l + j -97;

            // p[i] is capital
            if ((l>='A')&&(l<='Z'))

            {   //key is capital
                if((j >='A')&&(j<='Z'))
                    {
                        if(m1 <=90)
                            printf("%c",m1);
                        else
                            printf("%c",m1-26);
                    }
                // key is lower case
                if((j >='a')&&(j<='z'))
                    {
                        if(m2 <=90)
                            printf("%c",m2);
                        else
                            printf("%c",m2-26);
                    }    
            }

            // p[i] is lower case
            else
            {
                if((j >='A')&&(j<='Z'))
                {
                    if(m1 <=122)
                        printf("%c",m1);
                    else
                        printf("%c",m1-26);
                }

                if((j >='a')&&(j<='z'))
                {
                    if(m2 <=122)
                        printf("%c",m2);
                    else
                        printf("%c",m2-26);
                }
            }

            kalpha = kalpha +1;

    }
    else
    {
        printf("%c",p[i]); 
    }
}

printf("\n");    
return 0;
}