Steps to reproduce
The error is also thrown when the Null Target Layer's position property has any expression applied prior to applying the tracking data.
OS: Windows 10 v.2004 Build 19041.508
AE: v.17.5.1 Build 31
Thank you for reporting this bug. This issue is not new to the After Effects Beta build; it exists in previous versions of After Effects.
The problem is that the point tracker is unable to modify the keyframes of a property while the expression is enabled. It's definitely a bug, but you can work around it by disabling the expression before applying the tracking data.
Thanks, Tim. The error message is pretty clear, it's just the first time that I come across this bug. As a temporary fix, AE could toggle off the expression prior to creating the keyframes, that would be less confusing – especially to less experienced users – and provide a better UX.