When I tested your formula, I confirmed that an error of Could not fetch url: occurred. But, fortunately, when Google Apps Script is used, I confirmed that the URL can be requested using UrlFetchApp. So, in this answer, I would like to propose to use Google Apps Script. The sample script is as follows.
Sample script:
Please copy and paste the following script to the script editor of Google Spreadsheet, and save it, and put a formula of =SAMPLE("URL") to a cell. If the function name is not found, please reopen the Google Spreadsheet and test it again. This script is used as the custom function.
function SAMPLE(url) {
const value = UrlFetchApp.fetch(url).getContentText().match(/ダウンロード:.+/);
if (!value) throw new Error("Value was not retrieved.");
return value;
}
Result:
When above script is used, the following result is obtained.

Note:
- This sample script is for the current HTML of the URL of
https://www.photo-ac.com/main/detail/4465781?title=アゲハ蝶とヒャクニチソウ. And, when the structure of HTML of the URL is changed, above script might not be able to be used. Please be careful this.
References: