A Free team has one active project and one free real number. It also has a testing-number allowance. An action beyond those limits requires Pay-As-You-Go; an invitation to another member does not itself require a paid team.
Check the resource type and the selected team's billing mode. Only an owner or administrator can complete the team's upgrade requirements. Upgrading applies to the whole team and is irreversible. If the team is already Pay-As-You-Go but the same prompt appears, send support the action and displayed mode rather than creating another team to avoid the limit.