Your outer if is missing the braces:

if(isupper(ptext[i]))
{
        if ((((ptext[i]+k)%26)+52) < 65 || (((ptext[i]+k)%26)+52) > 90)
        {
            printf("%c", (((ptext[i]+k)%26)+78));
        }
        else
        {
            printf("%c", (((ptext[i]+k)%26)+52));
        }
}

Personally, I would extract some of the common elements into variables:

char something1 = ptext[i];
if(isupper(something1))
{
    char something2 = (something1+k)%26;
    if ((something2+52) < 65 || (something2+52) > 90)
    {
        printf("%c", (something2+78));
    }
    else
    {
        printf("%c", (something2+52));
    }
}

And maybe even put a char something3 = something2 + 52; in there too. Of course, with more meaningful variable names.

C is not like Python where it tells what belongs to what by indentation. Because in C, all white space is ignored. You need to use braces to tell the compiler which if statement the else belongs to.

The C compiler doesn't know, so it's asking you to specify:

Is it this?

 if(isupper(ptext[i]))
 {
            if ((((ptext[i]+k)%26)+52) < 65 || (((ptext[i]+k)%26)+52) > 90)
            {
                printf("%c", (((ptext[i]+k)%26)+78));
            }
            else
            {
                printf("%c", (((ptext[i]+k)%26)+52));
            }
 }

or this?

 if(isupper(ptext[i]))
 {
            if ((((ptext[i]+k)%26)+52) < 65 || (((ptext[i]+k)%26)+52) > 90)
            {
                printf("%c", (((ptext[i]+k)%26)+78));
            }
 }
 else
 {
            printf("%c", (((ptext[i]+k)%26)+52));
 }

This is an example of a "dangling" else.