How do I do /scoreboard players test functions in 1.14?

It sounds like you are using an impulse command block.

In your case it seems like you want a repeating command block, or you can build a redstone clock that activates your command block repeatedly.

You may want to build something that looks like this, to prevent the player from saying "hi" repeatedly:

You would have this command in the repeating command block:

/execute if entity @p[scores={totalKills=10..}]

And this command in the impulse command block:

/execute as @p[scores={totalKills=10..}] run say hi

An impulse command block activates ONCE whenever you give it a redstone impulse, or the moment you set it to "always active"

A repeating command block is blue, it activates repeatedly, as long as it has a redstone signal going in to it, or as long as it is set to "always active"

A chain command block is green, it activates whenever a different command block that points at it (there should be arrows on the command blocks) and is touching it activates and there is redstone going into it, unless it is set to "always active", in that case it does not require redstone.