undeclared identifier error occurs when I already have declared the variable

You've declared two variables in two different blocks, but then tried to use them outside of those blocks. Instead you want to declare one variable, and assign a value to it within each of the blocks:

int startWordLenRec(char s[]) { int length; if (isLetter(s) == false){ length = 0; } else { length = 1 + startWordLenRec(s+1); } return length; } 

(I removed the extraneous "if" from after the "else".)

However, a conditional expression would be clearer (IMO):

int startWordLenRec(char s[]) { return isLetter(s) ? 1 + startWordLenRec(s+1) : 0; } 

Move the declaration of length outside of the if statement. The way your currently have it declared it ceases to exit after the if statement executes so return length is always undeclared

Declare length outside of the if statements.

length needs to be declared at the top, since it is used in both branches of the if and outside (in the return statement).

You declared the variable "length" inside first and second if statements. Thus, it is not visible outside if statements