Validation query

Hello,

I have created validation on Opportunity product,

However the validation is when opportunity stage on (Contract Sent , Contract Signed , Contract Approved ) User not able to add or remove product and had created roll-up on Opportunity which represent that how many product have in Opportunity but I'm unable to proceed that cause of error please look into it snapshot is below:

AND(

OR(

ISPICKVAL( Opportunity.StageName , "Contract Sent"),

ISPICKVAL(Opportunity.StageName, "Contract Signed"),

ISPICKVAL(Opportunity.StageName, "Contract Approved")), Opportunity.NumberofLineitems__c <PRIORVALUE( Opportunity.NumberofLineitems__c ) )

)