The error is because Dell bios detects a fan rpm outside normal window.  Bios is written to monitor Dell fan only.  When a non-Dell fan is used, bios doesn’t know and thinks the fan has gone bad.  You cannot edit bios to get rid of this error.  It is a Dell built in safety warning.

if you replaced stock fan because it was broken or noisy, best solution is to replace w genuine Dell fan.

User anecdotes report certain non-Dell fan does not trigger this error.  It is by trial and error.

No guarantees, but Noctua fans have been a common replacement if not using a Dell fan.

@Anonymeus 

@bradthetechnut 

3020 7020 9020 fans are not wired the same as standard fans.

There is no bypass

Pocaton DELL FAN Adapter cable

I'm getting the "rear fan failure" alert at boot up too, but I'm still using the factory fan. I put in a USB 3.0 PCIe card that I ran a front 3.5" bay hub w/ USB 3.0 & various card readers from. I installed an SSD to run the operating system off of, but I left the factory HHD in as main storage and the big bulky case + fan. It's plugged into the motherboard and it seems to be operating fine. I get the "rear fan failure" at start-up, but after I start anyway it boots up fine. Is there a way to disable the alert or does anyone have an idea why it's there considering I'm using the factory fan? 

Since you have a 755 tower you should first replace cmos battery w a brand new one, if not already done.  Then clear cmos settings by motherboard RTCRST jumper.