internal drive "you can read only"

Hello all. I just “upgraded” my MacBook Pro (Retina, 13-inch, Late 2013, MacBookPro11,1) (from Sierra to High Sierra (10.13.2) and I am having permissions problems. On my Internal drive, I cannot drag and drop files anymore. For example, I have (on the first level of the Internal Drive) a file called “A.docx” and a folder called “B”. I can no longer drag and drop a file. For example, if I drag file “A.docx” into folder “B”, the original file will still be present where it was, and the file will also be copied into “B”. This is NOT how the Permissions were arranged before I “upgraded.”

When I did “Get Info” for the Internal Drive, I see, in Sharing & Permissions, that it says “You can only read.” The listed names (under Sharing & Permissions) are “System” (“Read & Write”), “Wheel” (“Read Only”), and “Everyone” (“Read Only”).

When I did “Get Info” for the “A.docx”, I see, in Sharing & Permissions, that it says, “You have custom access.” The listed names are “[UserName]” (“Read & Write”) and “Everyone” (“No Access”).

When I did “Get Info” for “B”, I see, in Sharing & Permissions, that it says, “You have custom access.” The listed names are “System” (“Read & Write”), “Wheel” (“Read Only”), and “Everyone” (“Read Only”).

Does anyone know how I can fix this? I believe I should be able to drag and drop any file as per the previous situation. Thank you in advance!!