Dear @Arman,
I suppose it's not a bug, since the same behaviour is already present on code.appinventor server.
Probably the mis-understanding arises for those (like me-italian and you-iranian) who aren't of English native language. I beieve that we should interpret the instruction sentence as:
<Project names cannot contain spaces. "space_test" SHALL be used, INSTEAD, to continue>
Dear Evan (@ewpatton), please amend me, if I'm wrong.
This is actually a regression in the current release. In previous versions of App Inventor, the new project dialog would accept the string and substitute the name accordingly. Thank you for the report @Arman. I have passed it along to one of our team members.