CS50 readability 2022

I am taking CS50x and in PSet 2: Readability throws me the following error when compiling:

Image of the Error message below, easier to read

readability/ $ make readability readability.c:52:24: error: multi-character character constant [-Werror,-Wmultichar] if ((txt[i] >= '97' && txt[i] <= '122') || (txt[i] >= '65' && txt[i] <= '90')) ^ fatal error: too many errors emitted, stopping now [-ferror-limit=] 2 errors generated. make: *** [: readability] Error 1

I think it has to be an issue that 97 and all other ASCII codes I am using are not recognized as integers, do I need to specifically declare them? If so, how?

Here is my code:

#include <cs50.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <math.h>

int count_letters(string);
int count_words(string);
int count_sentences(string);

float l;
float w;
float s;

int main(void)
{
    // Ask user for string, store in txt.
    string txt = get_string("Enter your text: ");

    int i = strlen(txt);

    // Convert letters and sentences to avg / 100 w.
    float L = 100 * (l / w);
    float S = 100 * (s / w);

    // Calc coleman-liau index
    int clindex = round(0.0588 * L - 0.296 * S -15.8);

    // Printf "Grade X" if X > 16, printf "Grade 16+".
    if (clindex < 1)
    {
        printf("Grade < 1\n");
    }
    else if (clindex > 16)
    {
        printf("Grade 16+\n");
    }
    else
    {
        printf("Grade %i\n", clindex);
    }

}

int count_letters(string txt)
{
    // Count letters
    l = 0;
    for (int i = 0, n = strlen(txt); i < n; i++)
    {
        // If the txt is between a-z (97 - 122) or A-Z (65 - 90), increase letter count.
        if ((txt[i] >= '97' && txt[i] <= '122') || (txt[i] >= '65' && txt[i] <= '90'))
        {
            l++;
        }
    }
    return l;
}

int count_words(string txt)
{
    // Count words
    w = 1;
    for (int i = 0, n = strlen(txt); i < n; i++)
    {
        // If there is a space (ascii 32), then increase word count.
        if (txt[i] == 32)
        {
            w++;
        }
    }
    return w;
}

int count_sentences(string txt)
{
    // Count sentences
    s = 0;
    for (int i = 0, n strlen(txt); i < n; i++)
    {
        // If txt is . (period 46), ! (exclamation 33), or ? (question 63), inscrease sentence count.
        if (txt[i] == 46 || txt[i] == 33 || txt[i] == 63)
        {
            s++;
        }
    }
    return s;
}

Thank you all for your help.

'' quotes are for single character literals.

'a' or 'd' for example.

'123' is more than one character literal.

You probably want to use plain integers instead:

(txt[i] >= 97 && txt[i] <= 122)