A repository can be left in an "invalid" state if
the repository was deleted outside of Tower.
was moved to a different path on your disk.
Since Tower can't find the repository files any more, you cannot work with a repository in such a state. Tower visualizes this invalid state by showing a yellow / red warning sign icon in the Dashboard or the "Repositories" toolbar button drop down.
Make sure the Dashboard view is currently open.
Right click the invalid repository and select "Relocate Invalid Path for <repository name>" or simply double click it.
In the following dialog you can choose to relocate the repository or remove it from Tower. If you want to continue working with the repository you have to relocate it by providing the correct path on your local disk.