Read through this checklist, and see below for the guides and walkthroughs.
✅ You need a Creator account on Instagram. Also known as business, or professional accounts. All the same thing.
✅ If you've just switched over to Creator account, you have to make at least one post first.
✅ Your Instagram profile must be connected to a Facebook page. See below for instructions on how to link, or create a page.
✅ You need to check your Facebook page is definitely connected! You can see when you tap Edit Profile in Instagram.
✅ Double check (maybe even triple check) that the Facebook login you're using is linked to the Instagram profile you're applying with.
✅ Finally - allow all permissions when you sign up on the page. If you uncheck any box, your application won't work.
Become a Creator Account on Instagram
Click on your own profile (bottom-right button) on Instagram, and then click the menu button top-right with the 3 lines (shown below):

Select SETTINGS. You'll see this (below):

Click on ACCOUNT. You'll see this (below):

Scroll to the bottom and you'll see Switch to Professional account. Tap it and it'll take you through some benefits of free professional accounts (below):

Follow the instructions on your screen. When prompted, select Creator or Business. You can select business if you like, we recommend selecting Creator (below):

Follow some final instructions and you're done. It doesn't change anything about your account other than you now have a few more options to select, and you'll be able to see your Insights - like Reach, Impressions and Engagement.
You're now a Creator! But you have to link your Creator account to a Facebook page. It's very quick.
Connecting your Creator account to a Facebook page
Connecting a Creator account to a Facebook is even quicker than becoming a Creator. Click on the 3-line menu again (top-right) and select SETTINGS again. This time you'll see an option for CREATOR. Click that and you'll see this (below):

NOTE: If you don't see an option to Connect to a Facebook page, you need to click on your profile, select Edit Profile and look here:

You should see the name of the Facebook page next to Page. If you don't, you haven't connected a page yet.
When you click Connect to a Facebook page, you'll see this (below):

If you already have a page, great! Just click Connect existing page and select the page. Most people don't have one - those people click Create Facebook page. Follow the instructions, give your page a name and you're done. Page created. You don't even need to fill the page with your details, once it's connected you're ready to go.
A final, very important point - if you've just become a Creator account, you'll have to post something on your profile as a Creator before you can apply for a program. This is to allow Instagram to register you in their system as a Creator.
Once you've done this, you're ready to apply for a brand program! If you've already applied for one and it didn't work, just apply again.