cannot add records join key of table error

[K0101]

Hello, I have made a form and on some of the text boxes it says cannot add records join key of table. Does anyone have any idea how I can fix this problem? Thanks

[CJ_London]

rule is one form, one table, your subform has three tables

I suspect subform recordsource should just be timeparked with car ID as a combobox with car and Pricing as it's rowsource