Well I didn't want to create a new environment as my previously created Models were in the default environment so I needed to assign my user a System Customizer role.
The Power Platform has its own roles which are separate to Microsoft 365 user roles and can be found in the Power Platform Admin Center.
1. Go to Power Platform Admin Center - https://admin.powerplatform.microsoft.com/environments
2. Choose Environments from the left menu and select the environment your working in.
3. Click the Settings option from the top menu.
4. Click the arrow next to Users + permissions and choose Users.
5. Select your user name.
6. In the top menu click on "Manage security roles"
7. On the right pane, select "System Customizer" and click OK.