Keep in mind this is Foreign Policy, the Deep State Fluffer Periodical, but it offers a plausbile explanation of why Russia's initial attacks failed.
TL;DR: Secrecy prevented specific trainings, logistics prep, and over reliance on Professional Forces meant there were no Follow On forces to pin down encircled or bypassed Ukie troops, leaving Russian supply lines and convoys easy prey for UAF with a few machine guns and light RPGs to destroy, forcing the lead units of the Schwerpunkt to retreat back with lack of supplies.
TL;DR: Secrecy prevented specific trainings, logistics prep, and over reliance on Professional Forces meant there were no Follow On forces to pin down encircled or bypassed Ukie troops, leaving Russian supply lines and convoys easy prey for UAF with a few machine guns and light RPGs to destroy, forcing the lead units of the Schwerpunkt to retreat back with lack of supplies.
https://www.foreignaffairs.com/ukraine/what-russia-got-wrong-moscow-failures-in-ukraine-dara-massicot