Heisenberg's Uncertainty Principle states that it is impossible to determine simultaneously the exact position and exact momentum of an electron.
It may not be possible to measure/determine position and velocity of an electron simultaneously, but practically an electron can have definite position and velocity then why it is stated as Bohr's model failure / drawback.

Who said it is practically possible.. its not possible

Aseem, Its moving due to the effect of internal forces and no external force is acting , therefore electron should follow a specific path. Although, it can't be determined, but a specific trajectory exists.

Tonty,
You should remember Newton's laws is not applicable to sub atomic particles 
Sub atomic phenomena is entirely dealt by quantum physics which is different from Newton's laws .So the laws for internal and external forces is not obeyed in sub atomic space.So Bohr's atomic model was said to be failure as it cannot explain multi electron systems 
hope the answer helps!!!
if you have any doubt regarding the same you can clarify here 
 

                                 HEISENBERG'S UNCERTAINITY PRINCIPLE'S VALIDITY                                                                  Ordinary experience provides no clue of this principle. It is easy to measure both the position and the velocity of, say, an automobile, because the uncertainties implied by this principle for ordinary objects are too small to be observed. The complete rule stipulates that the product of the uncertainties in position and velocity is equal to or greater than a tiny physical quantity, or constant (about 10^-34 joule-second, the value of the quantity h (where h is Planck's constant). Only for the exceedingly small masses of atoms and subatomic particles does the product of the uncertainties become significant.
The field of quantum mechanics concerns the description of phenomenon on small scales where classical physics breaks down.
The difference between quantum mechanics and newtonian mechanics is the role of probability and statistics. 
hope you understand now!!!

Sreeram, I want to explain that Heisenberg's uncertainity principle states that it is impossible to '"determine'" simultaneously , the exact position and exact momentum of a microscopic particle.
Yes, it is impossible to determine but the particle would have exact position and exact momentum or velocity which cannot be certainly measured by equipments which uses waves to determine the position or velocity of the particle.

The trajectory can be traced but u can tell where the electron will go in the next second.. so how u can tell practically

Aseem,
it's not about telling, it's about practical possiblity.
I am trying to say that trajectory of microscopic particles can't be determined(H U P) but it exists.

It can't be determined due to the measuring devices have to use some kind of medium , eg. light ( regular microscopes) , electrons ( in electron microscope)

Heisenberg's Uncertainty Principle states that it is impossible to '"determine"' simultaneously the exact position and exact momentum of an electron/microscopic particles. It may not be possible to measure/determine position and velocity of an electron simultaneously, but practically ( in real ) an electron can have definite position and velocity, which cannot be certainly measured by equipments / measuring devices, since they have to use some kind of medium , eg. light ( regular microscopes) , electrons ( in electron microscope) Then why it is stated as Bohr's model failure / drawback.??? Bohr told a mathematical way to calculate electron's velocity and radius of orbit, which can't be checked. Therefore his calculations can also be correct.