(SOLVED - ERROR 31)
I had the same issue. I have a middle name, but since in my passport it's usually stated together with the first name, I tried to put both together, only with the first letters capitalized (Xxxxxx Aaaaaaa Yyyyyyy Wwwwwww), but it didn't work. What worked for me was to follow the suggestions I found here:
1. Fill exactly like in my sin number letter (in my case, in CAPITAL LETTERS)
2. Not include my middle name (Aaaaaaaaa), as in my sin number letter this is not included (the agent only inserted my first name and family name, like the following example)
Prénom / First name: XXXXXXXXXXX
Second(s) prénom(s) / Middle Name(s):
Nom(s) de famille / Family Name(s): YYYYYYYYYYYYYY WWWWWWWWWWWWW
If none of this works, although it's not as convenient as the online submission, you can also print your tax return already filled and send it in person (https://www.canada.ca/en/revenue-agency/corporate/contact-information/where-mail-your-paper-t1-return.html).
Hope this helps.
