This might be occurring if Terraria can't update the achievements file. Please try the following:
Go to C:\Users\[Name]\Documents\My Games\Terraria
Look for the file achievements.dat
If the file exists: Right-click the file > Properties. Ensure the Read-only box is not ticked.
If the file does not exist: Right-click the Terraria folder > Properties. If the Read-only box is ticked or has a solid square, clear it and say Yes to applying the change of property to all files and sub-folders.
Then restart Terraria and check if the issue persists.
If the issue still persists, try running Terraria as an Administrator and disabling any anti-virus or file sync programs.