Good. so you are making some progress. This still points to a permissions issue .
Can you try the following test
test1: (you need to be sys admin):
1. change the 'write' permission on the Agreement request entity for this user's security role to Org level.
2. Now do the addition of a new Tranche entity record through the sub grid on the agreemtent request entity.
what is the result?
Let me know.