it’s for the year you had.
if you had a 18 and a 19 licence then you can use them on a 18 and a 19 versions. not on a 17 version, nor a 20.
in 2019 the subscription was introduced. unlike classic licences, you pay every year and have access to all the versions.
late 2021 the classic licence stopped being sold.
What about it? The classic license you have will only work with the version of SketchUp for which it was purchased. If that’s SketchUp 2019, it’ll only work with SketchUp 2019.
Yes u ryt but I try the same as u said which is to use sketchup 2019 classic license in sketchup 2019 but unfortunately it shows that my serial number and authorization code is not valid combination
solved here
by Colin.
For your interest, they had an unused 2020 version of the Classic license. They are up and running ok now in 2020.
I’m running into the same issue. I have regularly updated the classic licenses from 2018 to 2021. I kept an older version on the machine, but when I went to reinstall it on the machine with 2021, I got the same message. Is there any way to know what is happening with my licenses?
Older versions will continue to work on a machine they were installed on at the time, but you can’t activate an older version on a new machine. Your license was upgraded a few times, so that it’s now a 2021 license. You can install 2021 and activate it on a new computer.
I will send a message to you with your license, to make sure you’re using the right values.
Hi i have the same message “That doesn’t look like a valid Serial Number and Authorization Code combination”. I had issue with windows 11 update on my computer and after solving that i cannot add my license to sketchup 2018. I bought sketchup pro 2017 with updates untill 2019 but i was using mostly 2018 (because my vray wasn’t compatible with 2019). How can i go back to working on sketchup 2018? Please help 
What are the first few letters of the serial number?
I messaged them to ask for the email and serial number. No reply yet.