Your issue is that there is no relation/connection between the excel you read in and name as GroupA and the t.test you perform.
you either attach() the GroupA table and then use its variables freely (though this is not recommended for serious/reproducible workflows, rather for quick interactive work),
or you provide t.test with explicit knowledge that your variables are coming from GroupA
t.test(GroupA$Nervous,GroupA$Peripheral... etc.)
or you use the formula version of t.test where there is a data param you set to groupA
t.test(Nervous~ Peripheral, data = GroupA ....