The reason why your account is in Free Read-Only Mode is because when you Signed Up to Project Plan 365 you probably didn’t add billing information and didn’t actually start a Standard or Business Trial. To start a 30-day free trial please add your card information by following the steps from Sign Up to Project Plan 365 Online Help Article.

Note: Please know that you can always cancel Auto-Renewal so you wouldn’t be charged when the trial period expires.