Timeline App will only allow me to select a field that is "single select" as the customized field type

[Scott_Stephenso]

Hi,
I am attempting to create a Timeline App, but the Timeline App will only allow me to select a field that is “single select” as the customized field type.

Any suggestions on how to select a field that is customized as a “Single line text” or a field that has been customized using the CONCATENATE formula?

Thanks