This usually means that the registration code has expired and needs to be generated again, or that the code is being incorrectly entered.
If the code has expired:
Registration codes are valid for about one hour before they expire.
If your code expires, simply do the following on the thermostat to re-generate the code:
1) Tap the three horizontal bars on the bottom left corner of the ecobee screen to open the MAIN MENU
2) Tap REGISTRATION. If it says "2 Steps Left," you will first be directed to the Wi-Fi set-up and your code will be generated once Wi-Fi setup is completed
Write the registration code down and attempt to register again via the following method: https://support.ecobee.com/hc/en-us/articles/115006077347
If the code is being entered incorrectly:
Your registration code may contain letters that look like numbers or vice versa.
For example, the letter “I” may be misread as a number “1”, or the letter “O” may be misread as a letter “0”. The ecobee displays your registration code in uppercase, so a letter “I” may resemble a 1 and a letter “O” may resemble a 0.
If your registration code contains the letter “I” or the number 1, or the letter “o” or the number 0, try using numbers in place of letters or letters in place of numbers.