The *_syntex_fields functions are located in the node.direct.*
namespace, not in the node.* namespace.

See source code:

set_*

https://gitlab.lisn.upsaclay.fr/texlive/luatex/-/blob/master/source/texk/web2c/luatexdir/lua/lnodelib.c#L8683-8719


get_*

https://gitlab.lisn.upsaclay.fr/texlive/luatex/-/blob/master/source/texk/web2c/luatexdir/lua/lnodelib.c#L8721-8759


diff --git a/manual/luatex-nodes.tex b/manual/luatex-nodes.tex
index 42a4aedc7..9b72fdf2c 100644
--- a/manual/luatex-nodes.tex
+++ b/manual/luatex-nodes.tex
@@ -2490,9 +2490,9 @@ number, for a glue, kern, hlist, vlist, rule and
math nodes as well as glyph
  nodes (although this last one is not used in native \SYNCTEX).

  \startfunctioncall
-node.set_synctex_fields(<integer> f, <integer> l)
+node.direct.set_synctex_fields(<integer> n, <integer> f, <integer> l)
  <integer> f, <integer> l =
-    node.get_synctex_fields(<node> n)
+    node.direct.get_synctex_fields(<integer> n)
  \stopfunctioncall