Dear Aradhana
{g-sec=26084,g-node=26082} means that the error was generated on the section with ID 26084, which is output via the page or page reference with ID 26082.
de.espirit.firstspirit.parser.eval.Undefined only means that a non-existent variable is used.
"Cannot iterate over" indicates that CMS_FOR is used to iterate over the unknown variable, e.g. $CMS_FOR(for_var,myvar)$...$CMS_END_FOR$ --> myvar is unknown.
The problem can be caused by the fact that there is a typo in the template (wrong variable name) or the variable is calculated beforehand and something goes wrong during this calculation.
Tip: Via using "display preview errors" normally the exact place where the error occurred is displayed. The stacktrace there is also clickable.
Best regards
Holger