What is the problem?
My folgertech FT-6 printer has been running with Octoprint for 6 months or more. Zero problems. On Thursday I finished a print, shut down the system and went to bed. The next morning when I started the system Octoprint could not detect a serial port. It has not worked since.
Here is the message in Octoprint:
Changing monitoring state from "Offline" to "Detecting serial port"
Serial port list: 
Changing monitoring state from "Detecting serial port" to "Error: Failed to autodetect serial port, please set it manually."
Failed to autodetect serial port, please set it manually.
What did you already try to solve it?
Read through a number of similar issues and followed their solutions, nothing has helped. I tried 3 different cables, reloaded octoprint fresh, tried a different pi, updated to newest version of octoprint, reverted back to old version of octoprint, added specific serial ports in settings (/dev/ttyACA*,/dev/ttyACM*, etc.)
At this point I'm wondering if it's something wrong with the printer board and nothing with the pi or octoprint
Additional information about your setup (OctoPrint version, OctoPi version, printer, firmware, what kind of hardware precisely, ...)
Octoprint version tried 1.3.12 and 1.4.2
Octopi version - Version 0.17.0
Pi - Raspberry Pi 3 Model B Rev 1.2 - also tried on a Pi 4
Printer is using an Big Tree Tech SKR v1.4
I'm not a linux expert but I can type stuff in when connected via ssh.