Based on your screen shot it’s formatted with Master Boot Record which does not support partitioning. Reformat the Device to HFS+ or APFS and use GUID Partition Map instead of Master Boot record.
Another thing, what version of macOS are you running on your MBA? If it’s Big Sur or later you cannot use it as a Time Machine volume unless it’s formatted as APFS. In addition most volunteers here always recommend that the Time Machine volume be dedicated to Time Machine only, not shared with other data.
APFS formatted drives negate the need for classic partitioning as well. You can create as many volumes as you want within the main ‘container’. If you insist on using the SSD for two purposes then format it as APFS and create two volumes.
So what version of macOS are you running pn your MBA?