Solved KunenaScssPhp\ScssPhp\Parser:

Joomla 5.0
PHP 8.1
Kunena version:6.2.1
I have this error.
Thanks

Hello,

I have looked to it but i don't have this issue it don't goes at the line that he said

Do-you have set a  custom.scss file ?

Hello,
I don't want to hijack this thread, but I had this same error when upgrading from 6.2.0 to 6.2.1 while on Joomla 4.4 (without upgrading Joomla). 
The same parser error giving a 500 error. 
- I do have a custom.scss file.
- I also have a Gantry template. 
What I noticed is that the @mixin I had in Kunena's custom.scss caused the issue when upgrading to K 6.2.1, without any changes to the file.  I removed that @mixin portion and placed it in the Gantry template's custom.scss file for the entire site. 
That solved the issue for me, as Gantry did parse the scss file properly, and while the mixin was just for the forum, it doesn't affect the rest of the site. 
There could be a bug in the parser. 
Hoping this helps,
Tomás 

If I search locally for mixin on my test site
I mainly come across it a lot in akeeba backup and gantry
so my comment/question is, are all extensions all Up to Date?

In my case, yes. All extensions are up to date and compatible with J4.4 

Within the custom.scss file I added quite a bit of code. Amongst all that code was some @include and @mixin 
I removed the portions of both to see if either was breaking when parsing, and the @include instructions were fine, but not the @mixin ...  
So I removed all of that @mixin part of the code I had and the forum was back working. In order to have that back again (mainly color coding categories) I inserted that same code in the main template's custom scss file and it worked fine. Meaning all the color coded categories displayed how I wanted. 
So from that I deduce that

1. The code was written properly and
2. Probably, Kunena's parsing of the @mixin functionality has a bug, but it could be something other within my @mixin instructions, though I doubt it as it was quite straightforward. 

Hoping this helps,
Tomás 

Hello,

Is-it possible to send to me your file custom.scss ? Like i can try to reproduce it

Apologies for the delay.

The problem apparently is with my template