Re: string function not work in calculated field
Hi @JoeAu,
Could you please share more details about the configuration of the calculated field in the list?
Could you please share a full screenshot of the configuration of your flow?
Could you please share more details about the error message when the flow run failed?
I have made a test on my side to create a calculated field in the list as below:

The number field in the list is Number type field.
And I have created a flow as below:

The exression in the flow as below:
string(body('Get_item')?['Annual_x0020_Leave_x0020_Availab'])
The flow would run successfully as below:

Please share more details so we would try to provide a proper workaround for you.
Best regards,
Alice