Your first SCL block on an S7-1500: declare the FC interface, type the interlock in two statements, and read what each compiler error really means.
An S7-1500 upload with no project: Upload device as new station, what comes back with names and comments, what is gone, and the backup to take first.
S7 plc error codes as the CPU reports them: the number, what the text means, and the cause, including calls that break when an interface changed.
Step7 plc error codes with the number, what Step 7 means by it and what to change, from chdir and TBI errors to calls that fail after an edit.