Hey there laird80,
Thanks for reaching out to Apple Support Communities. I understand you’re having issues creating a custom label for some of contacts on your iPhone. I’ll be happy to help with this.
If you’re able to create a custom label for some contacts but not others, you may want to check which account the affected contacts are syncing with.
To do this, open the Contacts app and tap groups in the top left. You should see what accounts are syncing contacts to your device. If there are more than one, tap on the group and see if the affected contacts are all associated with this account. If they are, remove one of the contacts in question, then add it under your iCloud account. See if you are able to add the custom label as expected after this. Take a look at these links for more details on this:
Use other contact accounts on iPhone
Edit contacts on iPhone
If the issue continues with the contacts in your iCloud account, make sure your iPhone is up to date and test again:
Update your iPhone, iPad, or iPod touch
If you're still not able to add the custom label, let me know if this is happening on multiple devices and if you’re getting any errors or alert when this occurs.
Regards.