How to create a Microsoft teams to an existing team site?

affur5 

If your team site has a connected Office 365 Group then you can add a team using the teamify popup in the lower left corner of your site.
If you already removed that popup, you can reactivate it using PNP Powershell

Set-PnPPropertyBagValue -Key 'TeamifyHidden' -Value 'True'

Or open Teams and select "Join or create a team" at the bottom. Then select "Create a Team" and then "from a group or team" and choose "Microsoft 365 Group".

Alternatively you could use Microsoft Graph Explorer to create a team for your group.

1) Get the group id of your team site (That would be the group with the same name as your site)

GET https://graph.microsoft.com/v1.0/groups$filter=displayName%20eq%20'Example%20Group'

2) And then add a Team to that group

PUT https://graph.microsoft.com/v1.0/groups/{id}/team
Content-type: application/json

{  
  "memberSettings": {
    "allowCreatePrivateChannels": true,
    "allowCreateUpdateChannels": true
  },
  "messagingSettings": {
    "allowUserEditMessages": true,
    "allowUserDeleteMessages": true
  },
  "funSettings": {
    "allowGiphy": true,
    "giphyContentRating": "strict"
  }
}

(https://learn.microsoft.com/en-us/graph/api/team-put-teams?view=graph-rest-1.0&tabs=http)

If you don't have a group connected team site (i.e. you created the team site from the admin center and choose to create a team site without a group), then you first need to create a group for that site.
You can do that by 

1. Selecting the little gear in the upper right corner
2. Select "Connect to new Microsoft 365 Group"

Or by using powershell and the  "Set-SPOSiteOffice365Group" SharePoint Powershell Command 

(https://learn.microsoft.com/en-us/powershell/module/sharepoint-online/set-spositeoffice365group?view...)

Set-SPOSiteOffice365Group -Site https://contoso.sharepoint.com/sites/classicsite -DisplayName "Classic Site" -Alias "classicsite"