Why does it say I have insufficient funds available, when I have money on my account?

The reason it says you have insufficient funds is likely due to the fact that there automatically is reserved an amount on your account. The reserved amount covers you, so that you can afford to sell the share again, or sell the shares you already have.

Neither Lunar or Saxo have the possibility to see what the reserved amount is, it adjusts all the time.

If you either transfer more money to your investment account or buy less shares, the trade will go through.