[SalesForce] How to bypass validation rule for trigger in salesforce

The problem looks clear to me as you are facing a challenge with the small point in order of execution of salesforce.

To understand this you need to refer to order of execution in salesforce.

Triggers and Order of Execution

If you go to the link you'll find out in step 4 the custom validations are run once again after the before triggers which fails your logic.

STEP 4: Runs most system validation steps again, such as verifying that all required fields have a non-null value, and runs any user-defined validation rules. The only system validation that Salesforce doesn't run a second time (when the request comes from a standard UI edit page) is the enforcement of layout-specific rules.

To avoid the problem I would request you to not make the checkbox BypassCloseddateValRule, you created to bypass the validation rule to FALSE in the trigger.

You should do this change in a later step after which custom validations are not called, i.e. in STEP 7 (After Trigger) or STEP 11 (Workflow field Update).

Because custom validation rules will not be called after those steps.

STEP 12: If the record was updated with workflow field updates, fires before update triggers and after update triggers one more time (and only one more time), in addition to standard validations. Custom validation rules and duplicate rules are not run again.

I believe this will solve your problem. Let us know if you face any other challenge.

Thank You.

You cannot reference the related Account's fields while writing a validation rule on the Task. The Task object WhatID is polymorphic which means it could be refer to several objects. On the whatid at least you cannot reference related fields like you can with ownerid which is polymorphic as well

The only reliable option will be to make use of an Apex Trigger that prevents the creation of the Task.