Hi @Lucas Lu ,
Welcome to Q&A Forum!
In my tests, I can not reproduce your problem.
Here is my test:
1.[userA] has Contribute, Design and Edit permission on http://vemdum205vm/sites/echotest
2.[userA] has Full Control permission on subsite http://vemdum205vm/sites/echotest/chaina
3.[userA] also have Full Control unique permission on Site Pages library of subsite
4.[userA] can create a wiki page
It is recommended that you perform the following troubleshooting:
1.Check user's permissions: Make sure user has Full Control permission to subsite and Site Pages library on subsite
- (subsite) Site settings >> Site permissions >> Show these items
2.Recreate a new subsite for testing
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