From Salesforce documentation:

You can’t use long text area, multi-select picklist, Description fields, system fields like Last Activity, cross-object formula fields, and lookup fields in the field column of roll-up summary filters.

So what you want is not natively possible in Salesforce.

You can use this workaroud.

1. Create a new field on you child object named as "Duplicate Lookup Text" of type Text

2. Create a Workflow Rule, Created and Edited to subsequently meet the criteria, and criteria being the Lookup Field is not null

3)Create a Rollup- Summary on Parent Object for Count and use the filter criteria as Duplicate Lookup Text is not null

Voila Done.

Src: https://help.salesforce.com/articleView?id=fields_about_roll_up_summary_fields.htm&type=5

From Salesforce documentation:

You can’t use long text area, multi-select picklist, Description fields, system fields like Last Activity, cross-object formula fields, and lookup fields in the field column of roll-up summary filters.

So what you want is not natively possible in Salesforce.

You can use this workaroud.

1. Create a new field on you child object named as "Duplicate Lookup Text" of type Text

2. Create a Workflow Rule, Created and Edited to subsequently meet the criteria, and criteria being the Lookup Field is not null

3. Create a Rollup- Summary on Parent Object for Count and use the filter criteria as Duplicate Lookup Text is not null

Voila Done.

Src: https://help.salesforce.com/articleView?id=fields_about_roll_up_summary_fields.htm&type=5

From Salesforce documentation:

You can’t use long text area, multi-select picklist, Description fields, system fields like Last Activity, cross-object formula fields, and lookup fields in the field column of roll-up summary filters.

So what you want is not natively possible in Salesforce.

You can use this workaroud.

1. Create a new field on you child object named as "Duplicate Lookup Text" of type Text
2. Create a Workflow Rule, Created and Edited to subsequently meet the criteria, and criteria being the Lookup Field is not null 3)Create a Rollup- Summary on Parent Object for Count and use the filter criteria as Duplicate Lookup Text is not null

Voila Done.

Src: https://help.salesforce.com/articleView?id=fields_about_roll_up_summary_fields.htm&type=5

From Salesforce documentation:

You can’t use long text area, multi-select picklist, Description fields, system fields like Last Activity, cross-object formula fields, and lookup fields in the field column of roll-up summary filters.

So what you want is not natively possible in Salesforce.

You can use this workaroud.

1. Create a new field on you child object named as "Duplicate Lookup Text" of type Text

2. Create a Workflow Rule, Created and Edited to subsequently meet the criteria, and criteria being the Lookup Field is not null

3)Create a Rollup- Summary on Parent Object for Count and use the filter criteria as Duplicate Lookup Text is not null

Voila Done.

Src: https://help.salesforce.com/articleView?id=fields_about_roll_up_summary_fields.htm&type=5