Calculated field

I tried to do what you wanted to do but without the today function and a way to update it it won't work.

I would suggest a creating flow to do it.

Run the flow on a schedule, every hour as the trigger or what ever is needed

In the flow do the following:

1. Get items (List to check)

2. Apply to each (Value from List)

3. Add a condition using advanced mode: @empty(items('Apply_to_each')?['CheckoutTime'])

4a. In the Yes Condition put another condition that compares utchNow() is greater than Checkin DateTime

4b. In the No Condition put and Update item and set the status column to OUT

5a. In Yes of 4a. Condition add update item and set the Status column to IN

5b. In No of 4a. Condition add update item and set the Status column to OUT

I created this to test if it would work and it seems to work quite well.