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.
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.