https://www.quora.com/Why-doesn%E2%80%99t-Ukraine-join-NATO
Ukraine would like to be a member. However NATO has a rule which prevents membership if there is an ongoing conflict, because this would require all other members to join in the conflict.

The conflict is of course with Russia, who invaded and annexed Crimea, and then invaded Donbass region of East Ukraine and continues aggression with attacks almost everyday despite a ceasefire agreement.

But despite this, Ukraine participates in joint exercises with NATO, gets defensive weapons from members including USA. It seems inevitable that Ukraine will join someday, despite Putin’s objections. That is because Putin only invades non-NATO nations.

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Before the Russian aggression towards Ukraine, Ukraine was not able to join NATO because Russia opposed.
After the Russian aggression towards Ukraine (2014), Ukraine was not able to join NATO because Ukraine has no control over all its territories.
It is up to Ukraine now. If the Ukrainian Parliament (the Rada) writes off all the territories already in the mouth of the Bear as irretrievable, then Ukraine will be granted NATO-membership ‘the next day’