Welcome @pennies2pounds
When you buy or sell in the market there has to be a seller or a buyer for you to buy or sell from. Depending on the market capitalisation and how many trades are executed per day, there may not be someone selling or buying.
The smaller cap, usually cheaper shares tend to have lower trading volumes. According to Yahoo Finance, Harmony has a market cap of 214.011M and averages 651,820 trades a day. This is small cap. Google on the other hand has a market cap of 1.782Trillion and average trades of 1,892,819. Trades are far more likely to be rejected by the market if the companies are small cap and traded less often.
Trades are also sometimes rejected by the platform if the quote is way off what you are expecting to pay. Again, this is more likely to happen to small cap, highly volatile stocks. It can also happen more in the first and last 30 minutes or so of trading, so it’s usually sensible to avoid these, especially the opening, in my short experience.
There is more information here including some info on different order types if you are a plus member.