The message says that you have no stock in January (prev = previous period)
SAP has always 2 stock figures: current period and previous period
assume you had 1800 in January (previous period)
you had not posted the goods issue in January,
hence you still had those 1800 in the current period (February)
so technically you have 1800 in the january field and 1800 in February field
Now you goods issued with date of January.
this removed the 1800 from the January field, and since this stock exists only once physically the February field is as well zero after this goods issue.
now you reversed the material with a date of February
this means you added again 1800 but only into the February field, nothing into January because you did not use a date of the previous period.
the situation after this reversal is:
1800 in February field - 0 in January field
Now you try to post this goods issue again, using a January date.
since you have nothing in January, you cannot issue anything in January, and hence you get the M7021 message.
to do the goods issue you have to change the posting date to February now.