Which Sandisk card is it?
Is this Sandisk card blue? If so, you've got a Sandisk "standard" card (Class 4) and that card has a slow transfer speed -- to slow to keep up with video.
You need a Class 10 card... that'd be a Sandisk Ultra, Ultra Plus, Extreme, Extreme Plus, or Extreme Pro card.
2/07/2023: New product announcements!EOS R8 EOS R50RF-S55-210mm F5-7.1 IS STMRF24-50mm F4.5-6.3 IS STMRF15-30mm F4.5-6.3 IS STM