When I input The quick brown fox jumps over the lazy dog, the following program prints not a pangram. Yet, I expect s to be 26 and printf("pangram") to be executed. What am I doing wrong?

#include <ctype.h>
#include <stdio.h>
#include <string.h>

char findpan(char arr[]) {
    int i, j, count = 0;
    for (i = 0; i < strlen(arr); i++) {
        if (isalpha(arr[i]))
            count++;
    }
    for (i = 0; i < strlen(arr); i++) {
        for (j = i + 1; j < strlen(arr); j++) {
            if (arr[i] == arr[j])
                count--;
        }
    }
    return (count);
}

int main() {
    int s;
    char str[60];
    fgets(str, 60, stdin);
    s = findpan(str);
    if (s == 26)
        printf("pangram");
    else
        printf("not a pangram");
    return 0;
}

Simple Solution?

Here a Simple solution, I made the guess that you probably just want to know if it is or it isn't a pangram and so i've changed your function to a boolean one:

#include <ctype.h>
#include <stdio.h>
#include <string.h>
#include <stdbool.h>

bool findpan(char arr[]) {
    int i,j;
    for (i = 'a'; i < 'z'; ++i) { // goes through the alphabet
        for (j = strlen(arr); j > 0; j--) // goes through the arr[] 
            if (tolower(arr[j]) == i) // checks if the letter exists
                break; // breaks the inner for-loop if letter found
          
        if (j == 0) // if letter not found
            return false;  
    }
    return true;
}

int main() {
    bool isPangram;
    char str[60];
    
    fgets(str, 60, stdin);
    isPangram = findpan(str);
    
    if (isPangram)
        printf("pangram");
    else
        printf("not a pangram");
    return 0;
}

Explanation

'a' to 'z' represent the range of the lowercase letters in Decimal numbers, in the ASCII table:

for (i = 'a'; i < 'z'; ++i) 

tolower converts arr[j] character to lowercase and then compares it to i:

if (tolower(arr[j]) == i)

stdbool.h is introduced for the use of bool aka boolean:

#include <stdbool.h>