# \[AMPL 24990\] AMPL

**URL:** <https://discuss.ampl.com/t/ampl-24990-ampl/960>\
**Category:** Google Group Mirror\
**Created:** [February 25, 2024, 4:10pm UTC](https://discuss.ampl.com/t/ampl-24990-ampl/960 "2024-02-25T16:10:05Z")\
**Posts on this page:** 1\
**Showing post:** 2

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**Author:** ![4er](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.ampl.com/4er/32/530_2.png) [@4er](https://discuss.ampl.com/u/4er)\
**Post date:** [February 26, 2024, 3:16am UTC](https://discuss.ampl.com/t/ampl-24990-ampl/960/2 "2024-02-26T03:16:10Z")

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The most common reason for this error is as follows: You have used an AMPL `var` statement to define variable `Pg`, which is indexed over set `GEN`. But you have not told AMPL what the members of set `GEN` are.

If you need more help, reply with an explanation of how you are providing the data for your AMPL model — for example, by reading a text data file as described in [Chapter 9](https://ampl.com/wp-content/uploads/Chapter-9-Specifying-Data-AMPL-Book.pdf) of the AMPL book. Also, be sure to attach copies of your files. (You can click on the ![image](https://us1.discourse-cdn.com/flex019/uploads/ampl/original/1X/5d0af8abc312c025e5779697580e8b4e032a9ed4.png) icon to attach files.)

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