By chance, I was also looking for the original reference for some time. Checked Scientific American reference from 1996 cited above, but it does not seem to exist. The page 10, is nowhere and it has some other topics. I used this quote from Wikipedia a month ago for an editorial, which also appears in several books. I regret that I trusted Wikipedia and other books for this quote "in this field, almost everything is already discovered, and all that remains is to fill a few unimportant holes"
Only "Max Planck: Wege zur Physikalischen Erkenntnis. Reden und Vorträge, Band 1. Leipzig 1943" has the original version- right from the horse's mouth on page 128. I used DeepL translator to get the idea
„als eine hochentwickelte, nahezu voll ausgereifte Wissenschaft die nunmehr, nachdem ihr durch die Entdeckung der Energie gewissermaßen die Krone aufgesetzt sei, wohl bald ihre endgültige stabile Form angenommen haben würde. Wohl gäbe es vielleicht in einem oder dem anderen Winkel noch ein Stäubchen oder ein Bläschen zu prüfen und einzuordnen, aber das System als Ganzes stehe ziemlich gesichert da, und die theoretische Physik nähere sich merklich demjenigen Grade der Vollendung, wie ihn etwa die Geometrie schon seit Jahrhunderten besitzt“.
"as a highly developed, almost fully developed science which, now that it has been crowned, so to speak, by the discovery of energy, would soon have assumed its final stable form. There might still be a little dust or bubble at one or the other angle to check and classify, but the system as a whole is quite secure, and theoretical physics is noticeably closer to that degree of perfection that geometry has had for centuries."
Translated with www.DeepL.com/Translator