Imagine 1 million objects travelling in space, at a constant speed, along an imaginary line. They don't deviate from that line for the sake of this argument.
Now, object 1 has a velocity of $v_{1}=1$ km/s relative to a point in space.
Object 2 has a velocity of $v_{2,1}=1$ km/s relative to object 1, which means $v_{2}=2$ km/s when calculating the velocity related to the point in space we considered with object 1.
Object 3 has a velocity of $v_{3,2}=1$ km/s relative to object 2, which means $v_{3}=3$ km/s when calculating the velocity related to the point in space we considered with object 1.
Then I would assume that object 1 million as a velocity of $v_{1M}=1$ million km/s when calculating the velocity related to the point in space we considered with object 1. However, this goes against the principle that nothing can exceed the speed of light.
So:
1.) Where is the error in this reasoning?
2.) How can we talk about the existence of a maximum velocity when $velocity$ is actually vector-based measurement which changes with the reference we consider?