My Necchi model 931 is not doing any zig zag stitches nor decorative stitches only straight stitiches

If you haven't removed the top to clean and oil the cams then likely they are stuck, hence zigzag.

could you explain more about what the machine zig zag stitch is doing or not doing?

Necchi BF is a straight stitch machine
Necchi BU is zig zag machine
Necchi BU Supernova had cams for additional stitches

Since your machine is brand new, it is under warranty. Take it back to the dealer you purchased it from and have them repair it. If you try any repairs yourself, or by a 3rd party, that will probably void your warranty.

since you bought the machine at a state sale it would probably be a good idea to have the machine serviced.
you should be able to go from straight stitch to zig zag with out any problems.
if you move the dials or knob & the machine will not do a straight stitch then it needs to be checked

are you using a interfacing or stablizer with the fabric?
try replacing the needle
possible the zig zag gear is worn or damaged & needs to be repaired

First, make sure you select the straight stitch setting on the round dial. It is the one that has the dashed lines, looking like the stitch itself. Then, you may also want to be sure that the stitch WIDTH (dial with the picture that looks like a straight line gradually turning into a zig-zag) to the lowest setting, which is generally 0. With some machines, even after selecting a straight stitch setting, moving the stitch width can change needle position. Sometimes it will initiate the zig zag, other times it will move needle position left to right. Does that fix it?

If you have a zig-zag stitch on your machine, you have the capability of using a twin needle when using a STRAIGHT stitch. The width between the needles is the same as the width for the z-z stitch but you won't be using that stitch. Only the straight stitch.

not sure which model you have but you should have a stitch length control and a stitch width control. locate the stitch width control and set it to zero.

if you have done this and the machine continues to zig zag the zig zag mechanism inside the machine is most likely seized up. try lubricating all linkages inside and move the control lever a few times to see if it will free up.

if not you may need a tech to service the machine.

scott.

make sure the pattern selector is set for straight stitch first. If it is set correct then the machine may be "stuck" is zig zag mode. May need to be de greased and then re lubricated.