Starting with TestRail 4.0, we made test suites optional for projects. We found that for many projects, having a single test case repository and organizing your test cases via sections and sub sections works better than separating your test cases into different test suites. This makes it easier to start new test runs, filter your test cases and navigate between your tests. Because of this we added additional project options to use different suite modes and e.g. disable test suites for projects.
You can enable and use test suites by editing a project under Administration > Projects. However, we highly recommend using sections and sub sections in a single test case repository instead of separate test suites for most projects. You can also enable test suites for a project later in case you want to start using it then. You can also learn more about suite modes here (section “Suite Modes and Baselines”):
I hope this helps and please let me know in case anything is unclear!