Xcode 9: Could not attach to pid
This worked for me:
Edit Scheme -> Info -> Executable -> Ask on launch
Credits to @nastya-gorban's answer here
Update
After spending a considerable time with examples on Apple bug report, they basically disregarded the issue as using manual certificates is not "expected".
Long story short, if you don't have a business account and hence multiple developers on the same account, you should be fine with using the automatic signing and should not see the issue.
If you do have a business account with multiple users (which I found it breaks automatic signing), this is their suggestion:
We suggest that you use automatic signing for your debug builds and manual signing for your distribution builds.