differential ADC with a potentiometer in the positive input and grounded negative input
The data sheet on page 3 tells you this: -

Common-mode input voltage VCM = (AINP + AINN)/2 = 2.5 V typically


So if AINN is 0 volts then the only typically valid value for AINP is 5 volts.

You have a pot on AINP and with 0 volts on AINN this set-up won't work.

See also the notes on page 14 about how to drive the inputs - inputs need to be driven differentially and centred about 2.5 volts. See page 26 for circuit examples of interfacing to the analogue inputs of this chip.