I can't create a new 1:1 meeting with my colleague
The dropdown says 'You already have a 1:1 together'
Resolution
On the 1:1s page, clicking Set up new 1:1 will set up a new 1:1 relationship, not a new meeting in an existing relationship. Instead, navigate to the relationship you'd like to set up a meeting for:
Navigate to 1:1s and select the 1:1 relationship from the list.
Click Create Meeting OR Edit Settings if the 1:1 is recurring.
You can only have one recurring 1:1 meeting cadence with a given user (for example, a recurring weekly meeting OR a recurring monthly meeting, not both). Instead, Create an Ad hoc 1:1 if you'd like to meet outside your existing cadence.