Keep in mind that only one Spotify account can control the music with voice commands within the same Amazon account.
It's possible to add other Spotify accounts to the same Amazon account, but these won't be able to use voice commands. See below how to set up multiple profiles in your Amazon account.
It is however necessary to have your Spotify account connected to the Alexa app in order for the speaker to appear in the list of available devices on Spotify. It will not appear if your account is not configured.
Keep in mind that it's necessary to link a Spotify account to the speaker's manufacturer account - Amazon in this case, and this is required from both ends (Spotify and the manufacturer). Therefore, each Spotify account must be linked with an individual speaker manufacturer account and the speaker can only be used by the account that is linked.
If you'd like you can create anAmazon Household which allows you to have multiple profiles in the same account, and you can link your Spotify account to your own Household profile and use the smart speaker you want, even if someone else is using another one in the house.
Amazon Echo, Amazon Echo Dot, Facebook Portal / Portal+, and all Sonos players.