Simply delete line 505 of misc/Makefile. https://cgit.freebsd.org/ports/commit/?id=3b79ed749ee2a749ddd040d5e1875ab34253dd4e
In case there's any doubt, these are the error messages from Synth: Thu May 23 15:39:49 CEST 2024 Regenerating flavor index: this may take a while ... Scanning entire ports tree. progress: 58.26% culprit: misc/py-streamlit Scan aborted because 'make' encounted an error in the Makefile. misc/py-streamlit (return code = 2) Flavor index generation failed: ports scan
Fixed in commit 89694f30d7e743eff4789abede53c5d463f3456a https://cgit.freebsd.org/ports/commit/misc/Makefile?id=89694f30d7e743eff4789abede53c5d463f3456a