The project number controls which invoices from the accounts are linked to the project. A wrong number could give access to other people's invoices across the group. That is why only a global administrator can change the field.
How to do it
Find the correct project number from the accounting system.
Ask a global administrator to enter it. The field is called External project numbers and is found under Edit project → Settings, which you open from the three dots next to the project name.
If you don't have a global administrator who can do it, contact us in the chat.
Afterwards, check that the invoices come in on the project.
Good to know
The restriction protects invoice data. It isn't about you lacking access to the project.
Other fields may also be greyed out because of your role.
Don't enter a real project number on a test project. The invoices may then end up in the wrong place.