> Page size 1024

So, you have very small page size. Despite of you want to index two
VARCHAR(100) fields, max index key is greater than 200 bytes. Page
size of 1024 allows to have index key with max of 256 bytes. Anyway
page size of 1024 bytes is highly not recommended. It is very slow.
More - modern FB versions not allow to create database with such small
page size (while it would work with existing databases).

All you need to do - backup and restore with bigger page size (4 or 8KB)

Regards,
Vlad