Malawi Coin 2 Tambala 1984 Mint Error Or Not ?

If your coin were made of solid bronze, then one could conceivably tell the difference between a nickel plating and a chrome plating with a magnet; the nickel would weakly stick, the chrome would not.

Unfortunately, this coin should be made of plated steel, so it will stick strongly to a magnet, no matter what it's plated with. So no help for you there, I'm afraid.

I'm not sure anything less than a portable x-ray fluorescence spectrophotometer would reliably tell the difference between a nickel plating and a chrome plating on a plated steel object. Chrome plating, if done well, ought to look "brighter" than nickel, so if it looks the same as a coin definitely nickel-plated, then one could probably assume it is nickel, until proven otherwise.

Which doesn't really help in answering your question as to whether it's a mint error or a post-mint replating job. All I think we can rule out at this stage is a "missing plating" mint error, since raw unplated steel would (a) surely have gone rusty by now and (b) look quite different to a nickel-plated coin.