[lammps-users] Mismatched fix in variable formula

If you mean that you want to output the result of a compute
reduce via the dump command, that isn't possible.

Compute reduce produces a single scalar value by summing
all the elements of a per-atom array. You can't output a
scalar as a per-atom quantity in a dump command.
You can output it as part of thermo output, via the thermo_style
custom command.

I suggest you read section 4.15 of the manual which describes
the various kinds of output quantities and options.