Hello @James KEI , Thank you for reaching out to us.
Could you please help me, on which installation window of ad connect is it giving error? As i suspect it is giving error on the below window while installing...
If Yes, Then you need to check the account permission, If you will choose" create new AD Account" option then you need to provide any account from on-Prem AD and AADConnect will automatically add that to the enterprise admin and will proceed with the installation.
And if you will choose the option "existing ad account" option: you need to provide enterprise admin from the AD.
Let me know if my understanding is correct or above answer help you, else kindly share the screenshot from the installation wizard and we will take it forward. Thanks.
Reference article: https://learn.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-install-custom