If you are getting the error Error from Instagram: (#10) The user is not an Instagram Business it is because you have not switched your account to a Business Account.

The following steps have to be done from your Mobile Phone.
To switch your Instagram account to a Business one:

Go to your Instagram profile, in your Instagram App on your phone.
Tap on the three bars in the upper right corner and select Settings.

Select Account and then tap on Switch to Professional Account.

Specify your business category.
Choose Business and then tap Next to continue.

Only Business accounts are supported currently, not Creator ones.

Add your business information accordingly and tap Next.

Once you switch to a Business account, you will have to connect it to your business Facebook Page, from your PC by going here or from your Mobile Phone as shown below.
To connect your Instagram Business account to your business Facebook Page:

Go to your Instagram profile and tap on Edit Profile.
Under Public business information, select Page.

Choose which Page you would like to connect to, or Create a New Facebook Page.

Keep in mind that you need to be an admin of that Page before you connect to it. Check the settings of the Page to make sure you have all the necessary permissions.

Note: Now that you have a Business Instagram account, that is linked to a Facebook Page, you can connect it with Publer by following the steps shown here. If you are not able to see the Instagram account after following the steps as outlined above, please take a look at the article here to try and add the account.
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