with regards to the Meraki API - have you installed PostMan?
I'm pretty new to the meraki api and have be messing with powershell rather than python and found that Postman with the Meraki Dashboard API collection installed helps you to run various api calls without the need for all the convoluted script code around it.
once you've sorted out the parameters etc for the particular API call, you can then wrap the necessary code around it etc.
With regards to your code - if you run the api call in Postman, it fails with the values you have below with a bad request error or 404 error.
The t0 parameter needs to be a valid time value - if you remove it as a parameter, like so and do this in Postman it returns a list of clients
try the your code in python again, but remove the t0 line and just have either startingafter = "" or endingBefore = "" (aparently can't have both according to error returned by Postman if both included in the API call
e.g. - this fails:
let me know how you get on