Please select a value that is no more than

This message may be a browser error message, html5 validation on number inputs.

Is there a way to get a custom error message on single pages?

https://www.ads-software.com/support/topic/please-select-a-value-that-is-no-more-than/

I have customized the message on the page I have built here:
https://www.betadynepower.com/dcdc-converters/fsmm1s/

But if I go to the single product page, I just get the validation error.

Yes, that message is set by the browser. I just added an example here to easily customize that message. It will work on the single product pages and the cart page.

This example lets you add a link to the error message.