Running SQL Server Agent with domain account

Hello,

I am not well versed in SQL server stuff, but i've been given this task none the less.

We've created a domain account for a 3rd part vendor to use for SQL things. They intend to run the SQL Server Agent using this account.

They are having issues with it, so asked me to troubleshoot. To test, I am trying to install a new SQL instance, and using this domain account to run it, assuming that it will be able to try from scratch then suss out what they've done.

While installing the new SQL instance, and changing the SQL Server Agent user account to our domain account, I get the error:
Account provided for service does not exist. Provide an existing account name for the service.

Putting it to another domain account gives the same error, but changing it back to the default allows it to go through.

Is there something glaring that i've missed...