I have this code and N, K, and other constant have been declared previously

I get this error

Conversion to double from cvx is not possible.

please guide me

thanks

```
cvx_begin
variable a(N,K) nonnegative
gama0 = P * db2pow(row0) / sigma2;
R = zeros (K,1) ;
for i = 1 : K
r = 0 ;
for n = 1 : N
r = r + a(n,i) * log2(1+(gama0/(H^2+norm(Q(n)-X(i))^2)));
end
R(i) = r / N;
end
eta = min (R);
maximize eta
subject to
isequal(R >= eta , ones (K,1));
isequal(sum(a,2)<= 1 , ones (N,1));
isequal(a<=1 , ones(N,K));
cvx_end
```