hi! im gonna try help as best as i can!
firstly, i would recommend opening SMAPI and uninstalling through the app just to give you a fresh start,
next i would reopen SMAPI and install again following the instructions given/ available on the wiki, once you have SMAPI installed you need to open steam and find Stardew Valley from your library. right click, select properties and then go back to the SMAPI window. copy the entire line that starts with "C:Program Files (ect) INCLUDING all punctuation such as quotation marks at the beginning and end and paste it replacing whatever is currently in the box titled 'LAUNCH OPTIONS' this should link SMAPI and your downloaded copy of Stardew. As for mods nexus mods is your best bed but i assume you know that.
You need to locate your mods folder within your pc and open it. Then on nexus locate the mods you like and download the latest release, if you are playing 1,6 it won't work if its version 1.4/1.5. Also check under the requirements section for each mod and see if it requires anything extra such as SpaceCore or JsonAssests, if so then locate these on nexus and drag them into your mods folder alongside your chosen mods.
once all this is done then close SMAPI set up and your mods folder then open Stardew Valley as you usually would (probably through a desktop shortcut) and you should be all set!!!!
im sorry if this is all stuff you have done before but these are the steps i have taken and its worked great for me!
hope you manage to succeed and happy stardewing!! :D
