This usually happens because of how the website was done. It may be a bit technical, but you can enable pasting (but only works once, once you reload the page, the fix is gone).
Right click inside the input field for the password and select
Inspect element. It will open a new window with a new frame looking like this. There’s a block that is highlighted, that represents the current input field. If you look at the end of said block (or in your case, it may be somewhere in the middle or at the start of the block) there’s an attribute called
onpaste which holds a value of
return false;. You need to get rid of that. To do so, double click on its name, that will let you edit said attribute. After that, delete the text and press enter. You should now be able to paste your password.
If you cant find that, right click on the block and select
edit as HTML. Then try finding it one more time.
If you still cannot find it, then it probably is disabling pasting via js code, which is harder to deal with.
Disclaimer: DO NOT POST SCREENSHOTS OF THAT WINDOW, it may contain very sensible data (hence why I blurred out most of mine)