\[m=\frac{m_0}{\sqrt{1-v^2/c^2}}\]
tells us that the mass of an object increases with it speed, as long as \[m_0 \gt 0\]
.Increasing the mass of an object requires an increasing force to accelerate it. If a constant force is applied, then the acceleration will decrease with increasing speed. As the object approaches the speed of light, its mass grows without limit. Then the force required to accelerate it also increases without limit. For this reason no particle with a non zero mass can ever travel at the speed of light. That is not to say that no particle cannot travel at the speed of light. Photons travel at the speed of light, and in fact any massless particle must travel at the speed of light.