You have redefined "norm" as a variable in:
norm = 1./eta;
Afterwards you cannot access the function norm() directly anymore. The solution is easy: Do not shadow built-in functiony by variables, but choose a different name for teh variable.
By the way: you can find the reason of such problems easily using the debugger. Type this in the command window:
dbstop if error
and run your code again. When Matlab stops at the problem, check the used expressions:
which norm -all % <- this will reveal the problem
size(s1) % <- might be useful in other cases
class(s1)
s1
