RE: There is not enough space to insert correction lines
There is an issue with your Line No.
The Line No. is normally inserted using incremental of 10000. When you need to insert something in the middle, it will try to insert the Line No. in between.
Example:
10000
---> New Line here will be Line No. 15000
20000
If your Line No. is on incremental of 1, there is no space to insert it.
Check how your PO is created. If it's coming from a customized function, then you will need to ask your developer to change it to 10000 incremental.