I am working on the trailhead challange for Create a formula field that returns an image to indicate data quality. The challange lists

Sales Managers have asked for an at-a-glance solution to see completeness on leads. Create a helper formula field that looks at 5 key fields on the Lead object and evaluates their completeness, then a second formula field that references the helper formula and returns an image.
The helper formula field should be on the Lead object with a name of 'Lead Quality Helper' and a resulting API name of 'Lead_Quality_Helper__c'.
The helper formula should be of type Number.
The helper formula should evaluate the following 5 fields: Email, Phone, Company, Title, and Industry and return 0 if blank and 1 if not blank. The formula should then add all the values together to return a total value.
The image formula should be on the Lead object with a name of 'Lead Quality' and a resulting API name of 'Lead_Quality__c'.
The image formula should reference the helper formula, and return an image based on the number returned by the helper formula. The image formula should be of type Text. Note: All of these images are already available in your Developer Edition.
1 = /img/samples/stars_100.gif with alternate text '1 star'
2 = /img/samples/stars_200.gif with alternate text '2 stars'
3 = /img/samples/stars_300.gif with alternate text '3 stars'
4 = /img/samples/stars_400.gif with alternate text '4 stars'
5 = /img/samples/stars_500.gif with alternate text '5 stars'
If none of the fields are filled out, the default should be /img/samples/stars_000.gif with alternate text '0 stars'.
The 'Lead Quality' formula must be added to the Lead Layout page layout.

I have made my Lead_Quality_Helper__c, 
IF(ISBLANK(Email), 0, 1) + IF(ISBLANK(Phone), 0, 1) + IF(ISBLANK(Company), 0, 1)+ IF(ISBLANK(Title), 0, 1) + IF(ISPICKVAL(Industry ,""), 0,1) and it passes the syntax check no problem.

I have been trying to make my Leady Quality formula and I keep getting the error "Error: Field Lead_Quality_Helper__c may not be used in this type of formula". It will not let me save the forumla, making it not let me complete the challange. I have searched specifically for this error and have seen it referenced twice on questions asked here, but there was never a reply on how to fix it. Any help would be greatly appreciated. I have no idea why it's not working.
 

I am working on the trailhead challange for Create a formula field that returns an image to indicate data quality. The challange lists

Sales Managers have asked for an at-a-glance solution to see completeness on leads. Create a helper formula field that looks at 5 key fields on the Lead object and evaluates their completeness, then a second formula field that references the helper formula and returns an image.
The helper formula field should be on the Lead object with a name of 'Lead Quality Helper' and a resulting API name of 'Lead_Quality_Helper__c'.
The helper formula should be of type Number.
The helper formula should evaluate the following 5 fields: Email, Phone, Company, Title, and Industry and return 0 if blank and 1 if not blank. The formula should then add all the values together to return a total value.
The image formula should be on the Lead object with a name of 'Lead Quality' and a resulting API name of 'Lead_Quality__c'.
The image formula should reference the helper formula, and return an image based on the number returned by the helper formula. The image formula should be of type Text. Note: All of these images are already available in your Developer Edition.
1 = /img/samples/stars_100.gif with alternate text '1 star'
2 = /img/samples/stars_200.gif with alternate text '2 stars'
3 = /img/samples/stars_300.gif with alternate text '3 stars'
4 = /img/samples/stars_400.gif with alternate text '4 stars'
5 = /img/samples/stars_500.gif with alternate text '5 stars'
If none of the fields are filled out, the default should be /img/samples/stars_000.gif with alternate text '0 stars'.
The 'Lead Quality' formula must be added to the Lead Layout page layout.

I have made my Lead_Quality_Helper__c, 
IF(ISBLANK(Email), 0, 1) + IF(ISBLANK(Phone), 0, 1) + IF(ISBLANK(Company), 0, 1)+ IF(ISBLANK(Title), 0, 1) + IF(ISPICKVAL(Industry ,""), 0,1) and it passes the syntax check no problem.

I have been trying to make my Leady Quality formula and I keep getting the error "Error: Field Lead_Quality_Helper__c may not be used in this type of formula". It will not let me save the forumla, making it not let me complete the challange. I have searched specifically for this error and have seen it referenced twice on questions asked here, but there was never a reply on how to fix it. Any help would be greatly appreciated. I have no idea why it's not working.