The type of preposition to be used with spend depends on the context of the sentence.

As explained in “Invest In OR Invest On” article:

IN = inside.

ON = used to refer to a position/ process.

So, if you spend inside something, prefer spend in.

When you spend on a position/ process, prefer spend on.

Examples:

I am planning to spend in (inside) India.

The falling level of spending in (inside) the economy is leading to a recession.

They have decided to spend on hiring new employees.

Our library will spend on buying new books.