What is 9l,10 error and why is it happening?

The problem is in these lines of your select statement:

100008     RECORD KEY IS FILE1-KEY
100009     ALTERNATE RECORD KEY IS FILE1-KEY,

You have defined the alternate key to be the same as the primary key, and you're using a file handler that doesn't support this (ex: JIsam).
If you comment out the ALTERNATE line then you will no longer get the error.
If you need to use identical keys, you can use the c-treeRTG file handler, which allows this syntax.

To diagnose the problem, run your program with the following properties:

iscobol.logfile=log.txt
iscobol.tracelevel=63

In the resulting "log.txt" file you will find the file status along with the extended file status.
The line for this error was:

Oct 28, 2020 21.41.49.669 INFO: Unknown error FILE1 (FILE1.DAT) FS=[9l],EXFS=[108]: unknown error

This shows that the error reported in your error message, 91,10 is actually a 91,108. The 108 error is further identified in our KB for how to diagnose 9? Errors: How do I interpret file status 9? and other EXFS extended file status codes (e.g. 9i,105)? This article shows the following:

/* 108 */ "key already exists"

Indicating that the OPEN was trying to add a key that was already added.