Most likely you have the readns parameter on one of the analyses (for example the sp analysis) set to "spectre.dc". This tells whichever analysis it is to try to read the nodeset file as a hint for the DC algorithm that precedes the analysis (pretty much every analysis requires a DC operating point to start from, including an S-parameter analysis). If the file is missing, it will solve the DC operating point as usual - the file is just a hint.
The reason why it's not an error is that quite often you do things like this:
dc dc readns="spectre.dc" write="spectre.dc"
Which means the first time it will give you a warning, then write the DC operating point at the end of the simulation. The second run, the file will exist, and it will use that as a starting point. We try to make it not too critical if the file doesn't exist - since it is a hint.
If you really want it to error out, you can do that by adding this option into your netlist:
increaseSeverity options warning_change_severity=error warning_id=[CMI-2015]
And then it will fail:
Error found by spectre during DC solution estimation, during DC analysis `dc'.
ERROR (CMI-2015): Unable to open nodeset file `spectre.dc'.
No such file or directory.