Tenant cracked a shower door and is refusing to repair. The Tenant admitted to the landlord they cracked the door.The lease states the Tenant is responsible for any damages caused by Tenant, guest, relatives, or invitee.

Sue the tenant for damages in excess of the security deposit amount. Document the expenses and photograph everything as evidence.

This is why there's a security deposit - if the tenant causes damage that has not been repaired you take it from there.

Is or did the tenant vacate? If yes, then you take the cost of repair out of the security deposit. If the tenant is still residing in the apartment you give them notice that you are adding the repair cost to the next month's rent (or spread it out if you want). But inform them (in writing) that if its not paid you will consider them delinquent on their rent and subject to eviction.