Why this happens
Mermaid's flowchart grammar reserves call, click, and href as syntax tokens for the click-interaction directive (click nodeId callback "tooltip"). When one of those words is used as a bare node ID -- most often as an arrow target that was never given a shape -- the parser reads it as the start of a click directive instead of a node reference and fails.
The same class of failure happens with end, subgraph, class, classDef, style, linkStyle, and interpolate, except those fail even when the node has a shape attached (call[Label] survives; end[Label] does not).
How to fix it
Rename the node to anything that isn't a reserved word -- callNode, callStep, apiCall all work -- and update every reference to it, not just the one that failed. If the ID only appears once with no shape, giving it one (call[Label]) is enough for call, click, and href specifically.
MermaidPen's studio detects this automatically and can rename every occurrence consistently with one click.