%If %then do %end %else do

[InêsMaximiano]

%MACRO validate;
%if &A = 0 %then %do; 
%createtable;
%end;
%else %do; 
%if &B = 0 %then %do; 
%createtable;
%end;
%else %do; 
%don't create table;
%end;
%end;
%Mend validate;

Does this code work?

%if &A = 0 then &B = 0, so always create table

But it %if &A <> 0, &B can be 0 (and we want it to create table) or it can be <> 0 and we don't want it to create table. 

Because &A = &B + &C

Am I missing any %end or %else do?

[A_Kh]

It seems to me the problem was overcomplicated. If &A= &B+&C and you need the table only when &A=0, then there is no need for &B= condition. 
I would simplify it like: 

%MACRO validate;
	%if &A = 0 %then %do; 
		%createtable;
	%end;
	%else %do;
		%put NOTE: Condition is not true, table is not created; 
		%return;
	%end;
%Mend validate;

[Astounding]

Definitely a problem.  While unbalanced single quotes can create problems, this is a call to the macro DON

%don

The simplest version of this macro might be:

%if &A=0 or &B=0 %then %createtable;

[Tom]

This line is a problem. 

%don't create table;

Hopefully that is not in your actual code because it could cause your macro to emit unbalance single quotes which will "eat" a lot of code until it finds a closing quote.

You can test both A and B.

%if (&A=0) or (&B=0) %then %do;
  * code to create table ;
%end;

Or perhaps test B and C?

%if (&B=0) or (&B=-&C) %then %do;
  * code to create table ;
%end;

And if A or B might not be integers (1.5 for instance) then use %SYSEVALF().

%if %sysevalf( (&A=0) or (&B=0) ) %then %do;
  * code to create table ;
%end;

[Kurt_Bremser]

This code can't work because

don't

is not a valid name for a macro. Only letters, digits and underscores can be part of a macro name.