If you're working on computer with 4GB of RAM or lower, anything beyond fairly rudimentary videos will not be a pleasant experience. For working with larger videos I would say that 8 Gb of RAM would be a minimum. (b) Your computer has a decent amount of RAM. (a) You are using 64 bit Windows (which most versions of Windows 10 are) and If you are working with larger projects regularly you should upgrade to Camtasia 9 which is a 64 bit program (and can therefore use more RAM) provided that:

The message was therefore probably telling you that you were out of RAM, not disk space. There are limits to how much actual RAM a 32 bit program can access. If computers run out of physical memory they can use hard disk space as 'virtual memory', but there are limits to what they can do with that especially if you are working with 32 bit (older) software, which is what Camtasia 7 would be.

Disk space (which is what 150 GB would be) is how much space you have for long term storage of data. Memory (random access memory, or RAM) holds data that the computer is currently working with. Fmella's suggestion is a good one but also, don't confuse 'memory' and 'disk space'.

Techsmith camtasia studio 7