@DStefanov you likely need to do a ‘sweep unmixable’ because of having old non ringct outputs:

https://monero.stackexchange.com/questions/3508/what-does-the-sweep-unmixable-button-in-the-current-version-of-the-gui-do#3517

https://www.reddit.com/r/Monero/comments/8bilrs/comment/dx7in2o