If you have two Instagram accounts that are linked to the same email address, it can be a bit tricky to link the right one to TikTok. Here are a few things you can try to make sure you link the correct account:
Log out of Instagram on your device: Before you try to link your account to TikTok, make sure you're logged out of Instagram on your device. This will prevent the app from automatically logging you into the wrong account.
Use a different browser or device: Try linking your account to TikTok using a different browser or device. This can help ensure that you're not accidentally linking the wrong account.
Clear your browser's cookies and cache: Clearing your browser's cookies and cache can help prevent the browser from automatically logging you into the wrong account.
Remove the old account from TikTok: this article will help you how to remove account from Tiktok
https://techictalk.com/how-to-unlink-instagram-from-tiktok/
If you've already linked the wrong account to TikTok, try removing it from the app. To do this, go to your profile, tap the three dots in the corner, then tap 'privacy and settings', then 'linked accounts' and finally 'Instagram', and remove the account from there.
Remove your old account from Instagram: If you remove the account from Instagram, it will also remove from TikTok. However, keep in mind that if you remove the account, you will lose access to it and the account and its data will be deleted permanently and cannot be recovered.
Keep in mind that if you are worried about losing access to the second account, you may consider to change the email address of the first account to a new one different than the second one before removing it.
It's also important to note that TikTok may have certain terms and conditions regarding the use of multiple accounts, so it's a good idea to review their policies before proceeding