Your thermocouple is bad. About a three on the 1 - 10 DIY difficulty scale. Fairly easy to replace. You just need to know the length. This link has several.
Zero 36" Universal Replacement Thermocouple For Gas Furnaces,Boilers And Water Heater,Compatible With Honeywell,Mars,Johnson,Robertshaw,Penn,Johnstone, Johnson Controls Thermocouple
https://a.co/d/2KWAFjE