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Well, first, make sure autofill is turned on in Safari preferences.

For specific websites, if I recall correctly, go into your keychain via KeyChain Access and look for (iirc) "never remember". When found, delete the entry.
 
IIRC is shorthand for "if I recall correctly".

"Password not saved" are the entries you want. Delete those and next time you visit there, you should get prompted for "Save" (not all websites are configured in such a manner to allow it).
 
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