Hardness of Short Interger Solution in lattices Short Integer Solution ($SIS_{n,m,q,\beta}$) is defined as

Given a matrix $A \in \mathbb{Z}_{q}^{n \times m}$, find a non-zero vector $x \in \mathbb{Z}^{m}$ such that $A \cdot x = 0\mod q$ and $||x|| \le \beta $.

In paper Trapdoors for hard lattices and new Cryptographic Constructions, SIS is proved to be hard by reducing standard worst-case lattice problem approx. SIVP to SIS i.e., if we solve a random instance of SIS, then any instance of approx. SIVP is solved.

In paper On the complexity of computing short Linearly Independent Vectors and Short Bases in a lattice, approx. SIVP is proved to be NP-COMPLETE.

By this, can we say Short Integer Solution (SIS) problem in lattices is NP-Complete?
No, we cannot say that Short Integer Solution ($SIS$) problem is NP-Complete.

The results from those two papers are not directly related like that, because on the first one, the reduction is from $SIVP_\sqrt{n}$ to $SIS$ while in the second paper, $SIVP_c$ is proven to be NP-Hard for any constant $c$.

Until now, there is no proof that $SIVP_\sqrt{n}$ is NP-Hard and, since it has been proven to be in the intersection of NP and coNP, researchers believe that it is not NP-Hard.