Here are some info about the processes and prerequisites, usually the errors are related to permissions, so you should ensure to have validated the Domain Admin permissions.
To create a forest trust:
Open Active Directory Domains and Trusts.
In the console tree, right-click the domain node for the forest root domain, and then click Properties.
On the Trust tab, click New Trust, and then click Next.
On the Trust Name page, type the DNS name (or NetBIOS name) of another forest, and then click Next.
On the Trust Type page, click Forest trust, and then click Next.
On the Direction of Trust page, do one of the following:
To create a two-way, forest trust, click Two-way.
Users in this forest and users in the specified forest can access resources in either forest.
To create a one-way, incoming forest trust, click One-way:incoming.
Users in the specified forest will not be able to access any resources in this forest.
To create a one-way, outgoing forest trust, click One-way:outgoing.
Users in this forest will not be able to access any resources in the specified forest.
Continue to follow the wizard.
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