I never said it was impossible for them. After all, they did build deployable nuclear weapons.
Their flaw is that they considered it obsolete. Which could not have been further from the truth.
So, given the shortcomings of their government and decision making process (which was decidedly leftist in nature, and that isn’t a compliment) it was of course, wrong.
So they never made one.
We got decades of vital and important use out of those we built.
There was a lot wrong with Soviet decision making regarding technology. Rejecting microelectronics and computer science in 1950s is a great example of that.
I am not sure about Sr-71 equivalent and related bomber programs. Spacecraft are outright more practical for Sr-71 kind of missions.