What, if any, reasons prevented Ukraine from joining NATO?

Given the annexation of Crimea in 2014: one could argue that Ukrainian officials considered joining NATO between 2015 and 2021. What if any obstacles did Ukraine face to joining NATO during 2015-2021?

A Google search of NATO membership requirements returned:

• https://www.nato.int/nato_static_fl2014/assets/pdf/pdf_2016_07/20160627_1607-factsheet-enlargement-eng.pdf
• https://1997-2001.state.gov/regions/eur/fs_970815members.html