My code is not accepted

Carna gives two different errors and they need opposite fixes. Read the line under the Join button.

Carna tells you which thing went wrong, and the two messages mean completely different things. Read the line under the Join button before you retype anything.

"No invitation matches this code" #

The join screen showing: This invitation is not available: No invitation matches this code

The code does not exist. Either it was typed wrong, or it was never a Carna code.

Check the capitals first. a and A are different codes. This is the most common cause by a distance.
A code never contains 0, O, 1, I or l. Those five are left out on purpose so a code read off a whiteboard cannot be mistyped. If you are looking at what seems to be a zero, it is something else.
Do not count the characters. Codes are not all the same length.

"This code has already been used" #

The join screen showing: This invitation is not available: This code has already been used

The code was real and somebody has spent it. Most codes are single-use: one code, one person.

Retyping will not help. Ask your teacher or your school for a fresh one — and if you were handed a sheet of codes, take the next unused line rather than the one at the top.

Before you use a code at all #

Check that you need one. If your school sent you a username and a password, there is no code to enter — go to www.carna.app and sign in with those, without registering. See Join your school or class.

Where the box is #

The school picker with Join another school with a code

If you are already signed in, the code box is behind Join another school with a code on the school picker.