A through C

E-COMPUTE, TRANSPOSE not implemented
SIC, COMPUTE command. Self explanatory...
User action : If your variables are images, use the VECTOR$\backslash$TRANSPOSE command instead.

E-COMPUTE, Variable cannot be written
SIC, COMPUTE command. The output variable is defined ReadOnly.
User action : Probably a user error. Check your variable name. If it is what you wanted, it means you are trying to overwrite a protected program defined variable, and this is forbidden of course.

E-COMPUTE, Variable must be Real
SIC, COMPUTE command. The required actions operate only on Real variables.
User action : Define intermediate variables if necessary.

E-COMPUTE, Invalid OUTPUT variable dimensions
SIC, COMPUTE command on Fast Fourier Transform action.
User action : See Help.