I wanted to know if I can receive some help with an error I keep getting.

Error: Check that is.data.table(DT) == TRUE. Otherwise, :=, :=(…) and let(…) are defined for use in j, once only and in particular ways. See help(“:=”).

I am not sure what the problem is, but this is also the model specification I am trying to complete.
multipleMediation <- Treatment_seek1~ b1*M1 + b2*M2 + b3*M3 + b4*M4 + c1*X #Regression paths from IV to mediators# M1 ~ a1*X M2 ~ a2*X M3 ~ a3*X M3 ~ a4*X indirectAWARE := a1*b1 indirectAGREE := a2*b2 indirectAPPLY := a3*b3 indirectHARM := a4*b4 total := c1 + (a1*b1) + (a2*b2) + (a3*b3) + (a4*b4) #covariances M1 ~~ M2 M1 ~~ M3 M1 ~~ M4 M2 ~~ M3 M2 ~~ M4 M3 ~~ M4
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