This. It should never have been returned to you in that state. Whether there's air in the system (clutch hydraulics shouldn't even be opened up to replace the clutch), the slave cylinder isn't installed properly, or there's a clutch fork/release bearing issue, the car was there for a clutch. It better work right or the job isn't done.Bring it back.
STOP trying to get first gear until it's fixed. Your killing the Synchromesh!