What permission should be given to view pages , as its home page ( landing page )

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SharePoint2016

Hi,

I have given contribution permission to user,
but its not able to view pages ( home page ), as he logins to site white screen comes up

( using site and pages ) ,as i have placed custom web part on home page
( its check in ).

Solution Required: What permission should i give to user to view all pages( wheatear its site or pages ) , as its landing page using custom layout.

Overserved: if i give "design permission" it works fine, user can view those pages having custom we part ( landing pages ), but as i give contributor ,
he cannot.He can go to site contents, using URL like "---/_layouts/15" etc but when he open site using URL ,only white page ( blank screen ) comes up.

What I have tried:

I have designed/copy contributor permission by creating new group to place user there.