Being charged for C+
 

I have gotten used to free upgrades but just booked a recent flight and when I go to select a C+ seat there is a dollar amount associated with that change. Showing $26 to change. I thought as a DM I get comp upgrades. Plenty of seats available.

Is this a glitch in the Delta matrix?

Yes. https://www.flyertalk.com/forum/37354443-post771.html

has your flight been actually ticketed? If you're booked but the ticket hasn't been issued you can't upgrade yet. Also these C+ upgrades are limited by upgrade inventory, which does not always equal the number of unassigned seats, but generally the times I've seen this it's been because the ticket hasn't yet been issued.

Technically, as noted, it's conditioned on SU upgrade class bucket availability. While they have typically not held back any C+ seats from the SU bucket, there's the possibility they could do so at some point (just as they hold back FC seats from OU Medallion comp upgrade bucket). Basically, they stopped simply providing "access" to C+ seats close to a decade ago when they decided to make C+ seats book into separate fares, instead of just being extra legroom seats that booked in Main cabin fares. Also, if there is more than one companion on the same reservation as you, no one is eligible for C+ upgrades since you can only upgrade one companion.

Ticket has been issued.

Interestingly here is the upgrade pattern

GSP to ATL upgraded
ATL to TUL upgrade at a cost
TUL to ATL upgraded
ATL to GSP upgrade at a cost

Odd

Sometimes it takes a bit for the system to catch up/realize you are eligible for free C+. You can go into your ticket and remove the upgrade requests, refresh, and then re-select the upgrade requests to basically 'reset' it if something is stuck in the system.
Also seen where the website wants to charge but the app lets you pick a seat.

This happens all of the time for me when travelling to Canada. I am not sure why, but it takes 6-12 hours for it to remove the charge and allow me to move to C+ for free.

Same thing just happened to me on a muti leg. DM 1MM here. I travel every week. Two the legs let me select C+ at booking. The other two legs were asking for $ to upgrade. I gave it 24 hours and it still asked. I called the DM line and they seemed clueless as to why I couldn't upgrade. They did it for me on the spot.
Yes its a Diva/first world problem but it REALLY annoyed me.

It's a bug on the website. Use the app to select free C+ seats on all segments. Then go back to the website to request complimentary FC upgrades, since that function is broken on the app.

Yes, you now need to use two different platforms to actually manage your reservation. Very Premium.