I have set up a new level 11 bard and was trying to select the spells i wish to add for my magical secrets ability. For some reason the level 6 spell soul cage is not listed as an available spell and I can't see any reason why this is the case. Thanks.

I thought I had the option of changing my spells when I level up?

I thought I could change any of my spells when I level up?

Magical secrets count as a bard spell. But you can only replace a spell with a bard spell. So if you choose a different spell to replace magical secrets it has to be from the bard spell list.  Below are the relevant snippets from the features.

Spells 1st level and higher
Additionally, when you gain a level in this class, you can choose one of the bard spells you know and replace it with another spell from the bard spell list, which also must be of a level for which you have spell slots.

Magical Secrets
The chosen spells count as bard spells for you and are included in the number in the Spells Known column of the Bard table.

Ah makes sense, thanks for the help

I'm having this problem for my Level 14 Magical Secrets

You must be class level 11 and have purchased Xanathar's Guide to Everything to unlock the Soul Cage spell for selection.

For Bards you may only select the spell as a choice for Magical Secrets at class levels 14 and 18. You cannot swap a spell to Soul Cage it must be chosen only as a "Magical Secrets" option.

The spell I have a problem with is Crown of Stars

Crown of Stars is a 7th level spell. You will not be able to select this until your 18th Level Magical Secrets.

"You choose a spell of a level you can cast" 
I have a 7th level spell slot. 

Have you purchased Xanathar's Guide, or the spells from it?

I have content sharing enabled

Ok. Pulling up a test character Crown of Stars shows as an available option in the 14th level Magical Secrets. Since you have content sharing I just want to confirm the character you are trying to add crown of stars to is in the campaign? Shared content in the builder only works int he campaign builder, not your personal one. Outside of that, and your character bring the correct level, I would suggest posting a link to your character so mods can take a look.

I have raised a support ticket. In the meantime I have it made up as a home brew spell as a temporary workaround

Ok. If support is able to help you it would help the community if you post what the result was. In case someone else is having this issue it would be good for reference.

I understand you can't add spells that are higher than level 5, but it is also not showing up with paladin and ranger spells of that level. I thought that was one of the benefits of a bard, being able to master higher level spell techniques of those classes faster.

I promise you the spell is not on my available list. I personally, wouldn't have bothered you to post this if it was. I don't know if its just a bug for swift quiver. It's in the player's handbook and it's not under S alphabetically and I checked the rest of the list too.