Can NaOH and NaH2PO2 exist together in aqueous solution?
Your question: Can $\ce{NaOH}$ and $\ce{NaH2PO2}$ exist together in aqueous solution?

I assume what you meant is your textbook is saying when two separate solutions of $\ce{NaOH}$ and $\ce{NaH2PO2}$ are combined, there should be no reaction. If that is what you meant &quot;exist together,&quot; then your textbook is correct because there won't be an acid-base reaction since $\ce{NaH2PO2}$ has no &quot;acidic hydrogen&quot; contrast to $\ce{NaH2PO4}$ and $\ce{Na2HPO4}$ as Poutnik mentioned in his comment (more descriptive presentation would be $\ce{Na^+^-OP(=O)H2}$). The following diagram summerizes this point:

As diagram shows $\ce{NaH2PO2}$ (sodium hypophosphite) does not have any acidic protons to react with strong bases like $\ce{NaOH}$. The two hydrogens in $\ce{NaH2PO2}$  have attach ed to phosphorous atom in covalent bonds. And also showing is sodium bicarbonate, which has one acidic proton to react with $\ce{NaOH}$.