I was successful in installing Acorn & Sage on my work computer using WSL. Now, I am trying to complete an install on my personal computer which is running ArcoLinux. I do have the LAMP stack installed as well. Not sure if these details are useful at all but I am not finding a work around for this error message so far. It’s also late so maybe I need to look at this with fresh eyes tomorrow… 
Thank you!
What are the permissions of the folder you are in?
Also (could be wrong) shouldn’t you be a couple of parent folders up from where you’re running this command? Like in the Bedrock root folder (assuming you are using Bedrock)?
Thanks for the response!
I am installing without Bedrock and in the Installing Acorn instructions it states:
If you don’t use Bedrock and you are using a Sage-based theme, you can install Acorn with Composer from your theme directory.
I am interpreting that as the themes folder, but maybe I am misunderstanding.
Ohhh okay that makes sense.
You should be in your actual theme folder then. Like one folder deeper. currently you’re in app/themes/ you should be in app/themes/theme_name.
I just tried creating a theme folder with Sage, and got another permission error.
Please don’t post screenshots of error messages
Searching your original error has some helpful results such as php - Composer: file_put_contents(./composer.json): failed to open stream: Permission denied - Stack Overflow that shows some commands you can run to fix the permissions
