Staff cannot submit or clock out on their time card

The submit action does nothing, spins indefinitely, or returns a validation message that names no field.

A time-entry rule is failing rather than warning. A rule set to raise an error blocks submission; the same rule as a warning does not.
The worker has no valid assignment for the period. A gap in assignment history leaves the day with nothing to charge time to.
The payroll period is closed. Once closed, entries for the period are refused.
A required attribute became mandatory. If a layout changed, existing draft cards can fail on a field that did not exist when they were started.

Whether the failure affects one worker, one group, or everyone — this separates a rule problem from an assignment problem quickly.
Whether a draft card started before the change submits, versus a brand-new one. If new cards work, the problem is the old data, not the configuration.
The period's status, and whether the same action succeeds for a date inside an open period.

Ask one affected person for the exact time and their worker number. Server-side logs are searchable that way and guesswork is not.

If none of the above explains it, raise a ticket and include what you checked. Reading this article cost you no hours.