Can i Install FSL Manage package on my free developer ORG? How?

fsl package can not installed need help

how to get Field Service Lightning add-on license

When i am tryinig to enable my Field Service Lightning it wasn't shown and only work orders that was there.

I am trying to finish the challenge in first chapter of formulas and validation in beginner trail(developer) to calculate the number of days between today and last activity date on a custom object.when I click on check challenge! I always get error message saying formula is incorrect.

here is the formula I did. TODAY()- LastActivityDate.
can anyone help?