What did the Buddha mean by
"no truth can be spoken" in the Diamond Sutra?
The truth about the nature of reality is, strictly speaking, cannot be spoken, because reality is so multifaceted that it can never be fully and precisely expressed in words and concepts.
This is in line with the rest of Diamond Sutra which tries to give the reader a taste of the experience of Emptiness.
When all phenomena are done away with, all means of speaking are done away with as well.
Snp 5:6 • Upasīva’s Questions • Thanissaro Bhikkhu https://www.dhammatalks.org/suttas/KN/StNp/StNp5_6.html