How to sum amount for a specified month?

I want to sum amount_voucher based on month date voucher.

I believe your desired process can be achieved by utilizing the SUM function.

By using this function, you can calculate fields in a table.

Refer to the following help documents:

SUM/CONTAINS Functions: Calculations in Tables | Kintone Documentation Site

Setting Formulas | Kintone Documentation Site

Hopefully, this helps ~