Hi ScriptQuestions,
Note: 2020b does not support the script below very well yet.
You may try upgrading to latest version to take a try?
Please try using XF nlbegino:
https://www.originlab.com/doc/en/X-Function/ref/nlbegino
//Start the implicit fit with instrumental error, column 4 is the y error and column 2 is the x error.
nlbegino iy:=(1,3,4,2) func:=Allometric1 weight:=1;
nlfit;
nlend 1 1;