I have recently added a new root source css file for magento storefront by putting it in default_head_blocks.xml throught below statement.
<css src="css/custom.css"/>
Now I ran upgrade command
php bin/magento setup:upgrade
I am having the same custom.less file at below path of my custom theme.
app/design/frontend/myvendor/mytheme/web/css/custom.less
When I now try to run deploy command Magento is giving me an error
pub/static/frontend/myvendor/mytheme/en_US/css/custom.css): failed to open stream: No such file or directory
I don't know if this is a version issue or not but I already tried signing off static files from backend by setting
store>configuration>advanced>Developer>Static File Setting>Sign Static Files
to No
as well as trying to run the command again with admin rights.
Still, the error persists.
Best Answer
This error was actually created as custom.less file was blank and therefore magento was not generating custom.css file from it.
I don't know if it is bug or not as blank custom.css file should be generated from respective blank custom.less file.
Adding some css rules to it solves this issue.