Strange, it's usually the other way around. By default (with the Basic Advisor), any numeric variable should be set to Interval. If you use the Advanced Advisor, then if the number of levels for a numeric variable is less than 20 (by default), then it is set as Nominal, so not sure how a variable with that many levels is being set as Nominal (unless it is being read in as character instead of numeric).
If nothing else, you can use a Metadata node (from Utility tab) to change the level of the variable that you want to be interval.
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