I am also having problems installing some applications and getting the error:

Validation Error

Close

The operation cannot be completed: another model requires the record being deleted. If possible, archive it instead. Model: Follow-up Criteria (followup.line) Constraint: followup_line_followup_id_fke

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Hi,

Either you can try fixing yourself or report it to the author of the module, also make sure that the modules are added properly to the addons path, service has been restarted after adding the module.

Also you can try out the other accounting modules: https://apps.odoo.com/apps/modules/15.0/om_account_accountant/

Thanks