I signed up for a Developer Edition previously, so when starting this Trailhead, I logged into it and followed the instructions.

One of the early instructions is to install Agile Accelerator from the AppExchange.  I can go there and select it, but when trying to install, the instructions in the Trailhead say "At this stage, you’ll be asked whether you want to install it in a sandbox or in production."  However after pressing "Get it Now", I get an error message that says:

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Oops! Looks like you don't have permission to install apps.

To keep your Salesforce secure, only system administrators have permission to install apps or components. Please contact your Salesforce system administrator to install this app or component.

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I have checked my user in my DE account and it shows that my user is a System Administrator.

My username is: jpayne@trailhead.com

So I can't continue with the Trailhead.

Can you help?

We installed one managed package on our production instance that contains on permission set, but this packed permission set is not available in our sandbox instance, Please let me know if anyone knows the reason and this kind of behavior. 

Some months ago I successfully created a "Profile Setting" for my packaged application, but I cannot remmeber how I created it.

Any ideas?

PS: I have read the literature but am still confused about whether a profile setting will update existing profiles with additional/changed custom object permission.

I am trying to install Digital Sales Aid in the Sandbox but it keeps erroring out. I got the following errors. Has anyone faced this issue before?

Errors:
Entity is not accessible 
(DSA_Administrator) In Field: apexclass - no apexclass named DSAPostinstallscripttest found
(DSA_User) In field: apexclass - no Apexclass named DSAPostinstallscripttest found

I am doing "Create and Edit Lightning Components" on trailhead

even I set follwing in campingHeader.css , could someoen give me hint?

H1.THIS {
    font-size: 18px;
}