D::dvar

Details

This message is generated when a list argument in D has a length other than 2.
List arguments after the first argument in D are assumed to indicate multiple derivatives.
A multiple derivative is specified in D using a list of the form {x,n}, where x is the differentiation variable and n is the number of derivatives.
Off[message] switches off the message; On[message] switches it on. For example: Off[D::dvar].

The second argument does not have the proper form to specify a multiple derivative:

This shows a valid differentiation specification in D:

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