If Gradual Ability Boost optional rule is active, you are unable to increase ability for your Eidolon by GAB rules, only by pop-out widow that appears upon reaching corresponding levels (original Ability Boost rules)
Please note, that there is workaround by enabling manual entry for Ability Boost progression and manually change abilities each time you gain such boost.
Steps-to-reproduce:
Enable Gradual Ability Boost optional rule (In System Settings)
Create Character Sheet
Add any Eidolon Ancestry to newly-created charsheet
Level-up Eidolon to 2nd level
Attempt to add boost via Ability Boost option in character sheet.
If Gradual Ability Boost optional rule is active, you are unable to increase ability for your Eidolon by GAB rules, only by pop-out widow that appears upon reaching corresponding levels (original Ability Boost rules)
Please note, that there is workaround by enabling manual entry for Ability Boost progression and manually change abilities each time you gain such boost.
Steps-to-reproduce: