Noisneil is a pro 
If you want a single key as trigger, it can't work properly. There is a reason why letters are not typed "when released" by default. So either stick with keys that are rarely used, or add a modifier.
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My MacBook Pro is a M1 Max 2021
if I write "it' s" the s is one space behind.
I attached a screenshot to explain that I would like trigger a shortcut to launch Spotlight
with one key.
At my level, the solution of Frankb works just fine for me.
using non-alphabetic characters
- Trigger "is tapped" = letter remains letter.
- Trigger "is long pressed" = launch an app.
It's strange. @Frankb said the same thing, but it works fine for me and my mac is slower than yours. Go figure. 
You can experiment with reducing the timeout. 0.1sec might be more reliable than 0.2sec. To edit the timeout, click the gear icon.
Anyway, I 100% agree with @Frankb. Don't use regular single character keys as hotkeys in apps that you might also type in.
Just click on "Keyboard Maestro" and select the app you want to launch.
Thank you, master 
As Spotlight is not an app I can see find in the application folder,
in this particular case, I can't select it.
noisneil, the problem has nothing to do with your macros. And the explanation is perfectly logical. It is a kind of a system limitation. It would work fine if ALL key were always typed "when released".
This "solution" also does not work properly
- Trigger "is tapped" = letter remains letter.
- Trigger "is long pressed" = launch an app.
It's just faster and better hidden. But when writing really fast I notice it immediately.
Let's assume that the letter "s" is also assigned a longpress action. When the word "sure" is written, the following happens: "s" is typed "when released". But "u" is typed "when pressed". If there is enough time between "s" and "u", all is well. If not, the "u" is typed before the "s" and you write "usre".
That's why it's better to use a modifier or characters you rarely need.
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Are you trying to get a long-press to launch Spotlight?
If so, you don't need to activate an application; just simulate the Spotlight shortcut keypress.
I just realize that "Typed String Trigger" is also nice.
Typing twice or three time the same letter will give me a macro while still be normal pressed once.