Creating a new variable that is an average of two out of three measurements

Please do not post screenshots. People on this community are trying to help you, so you can try to help them to help you easily by sharing your dataset in a copy paste friendly format.

If you don't know how to do so, please share dput(head(CARRS)). It's better to produce a minimal reproducible example for your problem, which is nicely described in the following post:

Also, just for curiosity, how do you manage to get such detailed error messages in R?

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