Make sure you have no pending verifications on Instagram:

• Go to the Instagram app from your mobile.
• Click on Edit Profile.
• Scroll down to Public Business Information.
• Click on Pages and check if you have any warnings.

• Finally, make sure to review all warnings accordingly and try connecting the Instagram account again.

Then, check for the same thing on Facebook:

• Go to the Facebook page that you have connected to Instagram.
• On the left, scroll down to Settings.
• Then, go to Instagram.
• If you have any warnings, make sure to review them accordingly.
• Finally, try to connect to the Instagram account again.

Lastly, check to see if you have any alerts on Business Manager:

• The connection needs to be renewed by the Community Manager (the person who created the account) in the Business Manager Suite.
• If you are the community manager, go to the Facebook Business Manager account. If not, ask your Community Manager to do so.
• Once you sign in, you should be able to see the Alert (Review Error) there.
• Resolve the error following the steps accordingly.
• Finally, try to connect the IG account once again.