26 The things we have worked for probably refers to the pastoral and missionary efforts undertaken by the recipients of the letter in their own community and surrounding communities. This work would be “lost” if the opponents with their false teaching are allowed to proselytize unopposed.
27 The idea of a reward for Christians who serve faithfully is not common in the Johannine writings, but can be found in Rev 11:18 and 22:12.

In other versions

Watch yourselves, so that you may not lose what we have worked for, but may win a full reward.

Watch out that you do not lose what we have worked for, but that you may be rewarded fully.

Watch yourselves so that you don’t lose what we have worked for, but that you may receive a full reward.

Watch out that you do not lose what we have worked so hard to achieve. Be diligent so that you receive your full reward.

Watch yourselves that you do not lose what we have worked for, but receive a full reward.

Look to yourselves, that we lose not those things which we have wrought, but that we receive a full reward.

Public Domain