Again, you're ignoring the fact that the entire biosphere experiences co-evolution, This is why an invasive species is a problem. It was not part of the evolutionary arms race in that environment. It means that the highly specialized attacks that the plants and animals have against each other, might not work at all against the invader.

The billions of years of coming up with ways to make a living there, is actually billions of years of coming up with attacks specialized to exploit weaknesses in each other.

Take viruses as an example. They are often highly specialized to attack specific protein clusters on the outside of cells. Now, sometimes, through some random mutation, they can attack a slightly different protein structure and jump to a different host species, where they run rampant.

That said, alien viruses would likely not be an issue to either side. They're too specialized. Bacteria, on the other hand, that might pose a problem. But can be compensated for.

But other than that. Humans are very good at being an invasive species. We cheat. If there is anything edible on the planet, we'll find it. And the planet wouldn't know what hit it.

Again, the only way the planet's biosphere could keep us out, is if it was fundamentally incompatible. Like if it had an atmosphere without oxygen, or if every plant and animal had lead or arsenic in their biochemistry.