https://www.intel.com/content/www/us/en/support/articles/000087669/intel-nuc.html

This blink code indicates an error occurred during POST.

Immediately check the RAM and confirm it is the correct voltage memory for the NUC

Try:

• Remove and reseat RAM
• Make sure the contacts on the RAM and the NUC RAM socket are clean
• Check for a faulty RAM module by trying the RAM in a known good system
• Try a known good RAM module in the NUC
• If the Intel NUC has two memory slots
  • try using one (1) RAM module at a time, swapping it between the upper and lower slots
  • try using RAM modules from the same manufacturer part number and speed